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.
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.
I'm expeirmenting Mario on the demo. I manage to do a combo about 30-50% range (rarely are times a little over 50). But I'm not sure if I can call it a true combo since it depends on the characters (Megaman is really hard to get into my combo apparently).
Actually, if it's the combo I'm guessing it is, Megaman is technically not trapped in it at all. CPUs just don't realize they can spam Up B to escape it at almost any time, instead mashing jump and air-dodge. On Link (and idiotic CPU Megaman), I've been able to deal 60% damage out of a down throw. Villager, however, seems to take less hitstun than other characters in addition to being very floaty, so it's hard to do anything to him with it. Pikachu gets caught for a few hits, but he gets sent too high and escapes before I'm dealing enough hitstun for the UAir follow-ups.
