I've seen many youtubers continue to use both terms but I never really understood what they meant? Can someone please help?
Frame Trap: Frame traps can include things like low-to-the-ground strings, where attempting to air-dodge out of tumble would lead to you taking 20-ish frames of landing lag and a powerful follow-up attack. But if you don't air-dodge, they hit you again in the air. it can also involve situations in which you technically end up out of hitstun, but any possible actions are too slow to prevent the attacker from hitting you again.
String: A string is a sequence of attacks used to rack damage and cause pressure, but during which the foe escapes from hitstun for long enough to take some form of defensive action. Even a string can be useful though, as an opponent can end up in a situation where they have to make a guessing game. If they guess right, they can escape. If they guess wrong, they could end up caught in a combo or true combo, or simply take a very powerful finisher hit.