Labbing is the term in the FGC for hanging out in practice mode trying to do certain things over and over. More specifically, you can lab combos, counterplays, or just get used to a character's moveset.
Okay, let's break down each of those terms, in case you've never heard any of them. The FGC isn't monolithic of course, but there's some sense of community between all of the people who really pour their hearts into fighting games. Or, at least, I guess there is? It's like anything else that you'd call a "community" -- a tenuous connection exists between all of the people inside it, but a greater than average connection exists between some of its subsets compared to the population in general.
Counterplay has a similar meaning as in other strategy games. There's some scenario that you believe you'll encounter, and you want to practice against it. To do so, you can either have a friend practice with you, or use a scripting program like Eddienput.
A moveset is simply the moves that a character can use. Though, it can include more than what's listed in the game itself. More on that later, maybe.
A combo is a string of attacks that connect against your opponent within a certain timeframe. A 3-hit combo would mean that I hit someone three times before they were able to recover. I don't intend to go into the mechanics here, but fighting games have "hitstun" (the recovery time after getting hit), "blockstun" (the recovery time after blocking), and "recovery" (the recovery time after executing a move). Every move has startup frames, active frames, and recovery frames. Combos are possible because you can "cancel" the recovery of one move by immediately starting another.
Labbing combos is important because this is non-trivial. Not all moves will cancel into all other moves, so you have to see which ones work and which ones don't. In Guilty Gear Strive (the game I'm mostly focused on for now), there's a gravity mechanic so hitting someone while standing gives you more time to combo them versus hitting them from a crouched position. A lot of moves can't easily start a combo, and a lot of combos require these things called Roman Cancels as glue. A Roman Cancel is a Guilty Gear-specific mechanic introduced somewhere around Accent Core that can cancel the recovery of any move in exchange for tension (one of your main resources). So basically, when you couldn't normally keep a combo going, you can, for a price. (The price is usually worth it. It's better to use a resource to keep pummeling your opponent than it is to let them recover and maybe gain the upper hand.)
So the other day I spent about an hour labbing combos in Guilty Gear Strive as Bridget against Leo Whitefang. The opponent is only important because there are weight classes, which influence the aforementioned gravity effect. There's also the slight differences in hitboxes, but I generally neglect those. (I'd expect fewer people memorize hitboxes than frame data, and neither seems perfectly necessary. It's like history: you can remember the exact date and time that things happened, or you can say "the French revolution happened after the American one," and people will still probably Google it because they forget, and really who cares?)
Anyway, the reason I was labbing was partly to build muscle memory for a technique called Tiger Knee (TK). See, it's fairly standard among fighting games to have some special attacks mapped to quarter-circle forward and backward that are also "air-ok", meaning they can be executed in mid-air. Quick break to explain the common control notation used in fighting games: directions are indicated using the numbers on a numpad, like exists on your computer keyboard assuming it isn't tenkeyless. It's assumed that your character is facing right, so that 6 would move them forward, 4 would move backward, 2 would crouch, 8 jumps straight up, 7 jumps back, 9 jumps forward, and 5 stands still.
7 8 9
4 5 6
1 2 3
A quarter-circle forward is annotated as 236. Then you have the attack buttons, which in GGST are P (punch), K (kick), S (slash), H (heavy slash), and D (dust). Tiger Knee involves jumping a minimal amount and executing a special that uses 236 or 214. You do this by executing the jump before hitting the last button of the special. So if the move was 236S, you'd actually input something like 2369S, or maybe it's more like 236,9,S. (The timing is quite finicky, hence my need to lab it. It seems that the stick has to return to neutral after the 236?)
The move I was working on was Bridget's Stop and Dash, which is where she throws out a yo-yo. The usual options are 236S, 236H, 214S, and 214H. The 236 versions hit when they come out, and the 214 versions hit when they return. The S versions go straight out from Bridget, and the H versions fly out at a diagonal. If Bridget is grounded, then the H versions will travel at an upwards diagonal (in the direction of 9). If she jumps, then they will travel at a downward diagonal (in the direction of 3). With Tiger Knee, things aren't too different. It's more important for defensive reasons than offensive ones, I think. Because the jump is so short when executing a TK, Bridget (or any character) spends less time vulnerable.
Anyway, I didn't just do Stop and Dash over and over. I tried to come up with combos. I even recorded some. GGST can track every move you put into a combo and replay it for others or yourself, but interestingly, it doesn't note plain movement. What I mean is, half of all combos will involve dashing forward or airdashing forward. The only way to know that someone else's combo requires that is to watch the demo video.
So here's one combo I found: 2147H > 66 > c.S > 2D > (yo-yo hits) > c.S > 632146S > RRC > 623P.
The first thing is the TK H hit-on-return version of Stop and Dash. This places a yo-yo behind Leo's feet that will come back and hit him in like 5-10 seconds or something. I dashed forward into close slash range. (In Guilty Gear, the slash button will execute a different attack if you're far away versus if you're close.) Bridget's close slash hits twice if you don't cancel it, and the second hit kind of launches her opponent up. 2D is a sweep with hard knockdown, which means it takes longer to recover from than if it were a soft knockdown. When the yo-yo hits and the opponent is in mid-air, interesting things happen. There are a lot of different possibilities based on the initial conditions of where they were, where the yo-yo was, how high off the ground they were, what the timing was. It's really beautiful.
After Leo is hit by the yo-yo, I can hit him with another close slash, but only if the timing was perfect before now; otherwise, he drops to the floor, and even though it looks like my attack should hit, it doesn't. 632146S is one of Bridget's overdrives, named Loop The Loop. She summons an oversized Roger to hit someone a bunch with yo-yos somehow... There's a lot going on. What matters is 1. it takes 50% tension, 2. it hits many times for a lot of damage, and 3. it has a long recovery if you miss. RRC is a red Roman Cancel, which means not only does it cancel the (long) recovery of Loop the Loop, but it adds one more hit against my opponent. (It can be tricky to execute. It requires hitting the RC macro while an attack is active and hitting the opponent.) The thing is, Loop the Loop will keep executing, because Roger is sort of a projectile. Using an RRC (or a pink RC for that matter) just allows Bridget to move around freely before she normally could. While Roger is still walloping Leo, I walk forward a little and do Starship (623P). This hits 3-4 times and launches into the air. The total number of hits for this combo is at least 20.
So I sat there, trying this over and over. (I want to make that clear, that even though I'm writing this node, the process is very error-prone and time-consuming.) I hit the SELECT button every time I want to reset the positions and meters, which was every 30 seconds to a minute. Either I would miss the TK Stop and Dash and start over (because otherwise I would have to wait for the yo-yo to return), or I would miss the second c.S, or I would miss the Loop the Loop, or I would RRC early because this is a lot of steps to hold in working memory.
I've followed this whole process for many other combos, usually made by other people. One that caught my fancy was tagged "intermediate" by its author. It started with Bridget in the left corner, facing right, against Ky Kiske, and it ends with Bridget facing left and knocking Ky through the wall. c.S > 5D > RRC > 214H > 66 > c.S > 214K > j.P > (yo-yo hits) > 4 > c.S > 5H~H > 214K~K (this would normally be annotated as 236K~K, but I wanted to emphasize that Bridget is now facing left). Words can't describe how flashy and cool this combo is. Everything would go wrong in its execution. First, 214H has to be very fast, perfectly fast. It seemed to help to tap 1 during the RRC for some reason. 5D can be held, so care had to be taken not to hold it down at all. Sometimes I didn't dash up far enough, so the c.S was a f.S (far slash), meaning I failed the combo. It was hard to input Rolling Movement (214K) right afterwards. What would happen a lot is I would get all of these steps right, but I would mess up the timing so that Ky landed on Bridget's right instead of her left. Generally speaking, if he landed on the left, the rest of the combo was easy.
There are more combos, notation, and explanation here. If you're particularly interested in Bridget combos, Jais is amazing. Twitter and YouTube
(Now, I haven't really covered labbing counterplay. There's a reason for that: I haven't done it. Honestly, it seems pretty hard. Find another guide for that if you're interested.)
I know this was probably too much to understand in one sitting. This too is the point of labbing. Hanging out in training mode working on just one concept, or a small number of concepts, is the main way to learn in fighting games. GGST and games like GranBlue Fantasy Versus have mission modes that impart some of this knowledge, but there are always things that the games won't tell you. In general, you can search for the name of the game and "frame data" to find a good resource. "guilty gear strive frame data" will take you to Dustloop, while e.g. "blade strangers frame data" will take you to Mizuumi. Games like Super Smash Bros. and Tekken 7 have their own sites. This place has a great list of other fighting game wikis, as well as info for Marvel vs Capcom: https://wiki.supercombo.gg/w/Main_Page. None of this is to say that you need to learn frame data, but rather that any wiki that shares it will generally have most of the other information you would want. And if they don't, maybe they link to a Discord. (https://wiki.supercombo.gg/w/SuperCombo_Wiki:Community_portal/Discords/Game) If you'd like to learn more about fighting games in general, this is a good guide.