It's when you press 2 or more buttons quickly and get a combo, like Ryu's heavy punch + heavy kick and Luke's 3x medium punch.

@@rodrigosantana9199 oh like ryus medium punch, light kick, heavy kick. then what is a string?

@@mendeleyev1790 Yes. A string in fighting games is a sequence of buttons. A good string Ryu have is crouching light punch, crouching light kick, crouching light punch and confirm into medium tatusmaki. It catches your opponent trying to walk back on wake up.

If you understand Tekken, I'd say that it is a natural combo. A combo that once it hits, it'll keep hitting. No mixups. Luke's 4x jab combo is a target combo.