oldwaysx Hitstop frames are when the game "freezes" upon touching the opponent. The amount of hitstop frames can vary depending on the game.
Hitstop is important because it can give you extra frames for you or the game to recognize whether a hit has landed on the opponent and to confirm that into something bigger. When a move whiffs, no hitstop occurs. Even more importantly, hitstop can open up opportunities for option selects based on whether a move hits or whiffs.
Street Fighter 4 was filled with these hitstop option selects but most games have these in some capacity.
It's okay if you don't play sf4, it's not particularly important. Rather what is happening. A move causing hitstop vs a move whiffing (no hitstop.)
Ryu jumps in with a j.HK and buffers in Ultra as he comes down. If Honda blocks it, the hitstop prevents Ryu from landing and does not activate ultra. If Honda does anything that makes Ryu j.HK whiff (backdash, ex headbutt, etc), the buffered ultra will come out since there was no hitstop to freeze the game and prevent Ryu from landing right away.