In other fighting games, frame traps refer to you doing a seemingly unsafe move that is actually safe, tricking your opponent into throwing out an attack that you can easily counter.
Smash has a different definition of frame traps, however. In Smash, frame traps refer to inescapable attack strings. For example, you hit someone into the air as Yoshi and throw an egg at them. They airdodge, but now they are falling into, say, Yoshi's f-smash because airdodges have vulnerable recovery frames. There is nothing they can do to avoid it at that point.