What is a frametrap? The term is very specific and we enter the more advanced mechanics and techniques now in my blog series. A frametrap is basically a trick to bait your opponent into trying to interrupt your pressure. What he doesn't know is that it was part of your plan and you have the advantage after your attacks.
Let's do this with a simple example where the lesson about framedata factors in. It's a Ryu vs Ryu mirror match in SF V. You do several attacks and end your attack string with a crouching light kick as Ryu. It's +1 on block. If the opponent doesn't know this, it's a big deal. All throws in SF V have a startup of 5 frames. With a frame advantage of 1 frame the opponent only has few options: he blocks or needs a move with 4 frames or less of startup, thows are not an option because your moves as Ryu would interrupt him. Does Ryu have appropriate moves for that situation? Yes. His standing light punch is 3 frames and his crouching light punch and standing light kick are 4 frames of startup. But you have the advantage. You can always beat his attempts with your own moves as Ryu or if he blocks you can throw him. Frame traps are a trick. You do several attacks and give the opponent the illusion that it's his turn now. But it isn't. It never was. There also comes another factor into play: Range.
Some moves are very good not because of their frame advantage on block but their range. Chun Li's standing heavy punch is -5 on block but it has excellent range and is hard to punish because most counter moves are too slow for that range after her attack is blocked.
Frametraps are really useful tools. If your enemy doesn't know that Ryu's crouching light kick is +1 he might be tempted to push a button. If you are ready and know that and push the right button yourself (a move in range with good startup) you will get a counter hit. Counter Hits are very important in every fighting game. There are combos which are only possible with a counter hit as a starter. Counter hits grand you often more damage and more frame advantage on hit.
What you should take away from this lesson is that frametraps, range and counter hits play a big role overall in fighting games. They are important for the ground game and might be what gives you a competitive edge over your opponent. Frametraps are essentially a trick to make the opponent push a button. It wasn't his turn and you can profit from it. Sometimes you just have to lie, cheat and steal to win. :P