In video game terminology a top loader is the newer redesigned version of the Nintendo Entertainment System that was released in the early 1990s.
The original Nintendo Entertainment System (or NES) was designed to look more like a VCR than a gaming console. So Nintendo came up with a complicated front loading cartridge mechanism that involved loading cartridges in way that was extremely similar to the way VHS tapes are loaded into video cassette recorders. They felt that this esoteric loading system would help disguise the fact that the Nintendo was pretty much just a fancy version of the Atari and Coleco systems that all the retailers had just lost a bunch of money on.
The fancy front loading mechanism meant that the NES had to use a strange zero insertion force cartridge socket. They didn't actually test this design all that well, and in real life these cartridge sockets had high failure rates after just a few years of use. You see that zero insertion force socket wasn't really a "zero" insertion force socket, and over time the cartridges would bend down the contact pins and wear out both the socket and the cartridge. Any dust or dirt would quickly multiply these problems. The whole setup worked great with brand new NES units and brand new cartridges, but it didn't work so well two years later when handled by sticky fingered children.
Unfortunately the cartridge socket wasn't the only thing conspiring to make these NES units fail early. You see each and every one of the original NES units contained a 10NES lockout chip. This was a security chip that mated up to an identical chip contained on each cartridge. The system required these two chips to talk to each other before it would boot. It was pretty picky about the timing and voltages. Now what happens when you try to be picky about timing and voltages over a poor connection? The red blinking light happens. The Nintendo Entertainment System would go into a constant reset loop and the user would be unable to play their game. The use of the Game Genie device would quickly accelerate this issue until the NES would not work without the Game Genie. But there was one plus, if you could get the Game Genie into a spot with a good connection, then you could simply insert and remove games from the Genie and they would boot every time.
During the early nineties Nintendo finally decided to release a NES unit without these problems. They had little to lose at this point. Tengen had long ago copied their security chip, other unlicensed game makers used a voltage override trick to bypass the security chip, while HES brand games in Australia actually required you to insert another cartridge into a dongle so the NES could talk to the security chip on that cartridge. Essentially the whole security chip business did nothing but hurt people who bought legitimate games.
While they were redesigning they put the cartridge slot on top of the unit, like a Sega Genesis or Atari 2600. That minor design change completely fixed the problem of older NES units being unable to load games. The top loader NES units look a lot like a small SNES and come with controllers that look similar to SNES controllers.
Top loaders tend to sell for very high prices on eBay, and it may be worth your while to pick one up if you are a NES fanatic.