One of the most frequently misinterpreted concepts in the whole field of writing — which is saying something.
The usual idea of the Gun is that it is a dictum on dramaturgy — basically, about foreshadowing, which is patently untrue. This you can glean simply by examining it properly; it goes thus:
If there is a gun hanging on the wall in the first act, it must be fired by the third act. Otherwise it shouldn't be there.
Now, this is usually read as meaning something like »You must put a gun on the wall, then have it fired by the third act«, where of course the gun is a metaphor for any dramatic planting or foreshadowing. Does this interpretation make sense? Think about it for awhile.
No, of course not. If that were what was meant, why would it say »if there is a gun«? Surely then it would say, as I have it above, »you must put a gun«? And why would Anton Chekhov, an overrated but at that point fairly experienced playwright, feel the need to assert that »you must have events in your play! They must occur, and then later have ramifications or resolutions«? Who on Earth needs to be told that, except maybe Chekhov himself?
The other idea, as outlined in the writeup above, is that it means you shouldn't put details in your story if they're not going to be meaningful later. But why would anybody make that commandment at all? Why repudiate all red herrings and other dramatic misdirection? Isn't that essentially identical to openly embracing predictability? Should all fiction be pervaded by the ineluctable doom of a Greek tragedy, where everyone in the audience knows the outcome going in? Ought we have clapped Dame Agatha Christie in irons, and dragged Doyle to the scaffold? Is a mentally overheated fan sensible and justified in constructing tottering edifices of speculation atop every inconsequential detail of the seven-book Harry Potter series and then getting irate when they turn out to be wrong?
As you can see, none of this holds up to five minutes' scrutiny; the idea crumbles like wet sand in the hands of an eager child. But what is it, then? Let's set the stage, not forgetting our ironic flourish:
In Russia at that time, there was a maximalist trend in stage decoration — this was in Imperial Russia, remember; Anton Chekhov died before many of the Bolsheviks could say »mama!« — whole lavish interiors of palaces scrupulously recreated or invented, a souk in Baghdad with fresh fruit in the stalls for every night's performance, a realistic railway platform, that sort of thing. People loved it! And it was possible, because of course in those days there were no televisions, theater was the best you could get, so that wealthy aristocrats poured money on it to make it as fun as possible to not watch while you talked to your friends and decided which actress you were going to sleep with.
Recall at this moment the Gun, hanging on the wall.
Chekhov, unlike most of his peers, was of a minimalist bent; he was a precursor asshole of those assholes we call modernists. He felt that ideally there should be absolutely sweet fuck-all besides the actors on stage, and if you absolutely must put any kind of scenery or décor on there, it should be vital to the plot. That is: If there is a gun on the wall, then it must be because the gun is going to be fired; otherwise it does not belong on stage — anything less than dramatic relevance is unacceptable, it can't just subtly imply something about Uncle Volodchek's personality. This of course is also an astonishingly stupid idea, just immensely awful — in fact, it's quite nearly as ludicrous as that notion about red herrings, earlier — and nobody should take this advice. But it is coherent advice. It makes a statement that is internally consistent and does not have a #NULL content value.
Unfortunately, the whole thing appears to have been misunderstood quite rapidly after Chekhov's death, and it's been fixed in its present »unmeaning nonsense« form ever since. But for fuck's sakes: you shouldn't repeat it.