Most famously seen in Killer Instinct, a combo breaker is any move that allows you to escape out of hitstun. Of course, you can only perform one while being hit.
Not all games refer to them as combo breakers. In Arc System Works fighters, they're most commonly called Burst. And while it is a separate mechanic, it can function as a combo breaker. Marvel vs. Capcom 3 allows you to escape air combos when your opponent tags in a character, called aerial exchange counters. Dead or Alive has defensive holds, which are used to interrupt a combo with a Geese-like counter technique.
Being able to escape from a combo either requires you to sacrifice meter, or win some kind of guessing game against what P2 is doing. Depending on the game, you may or may not be able to inflict damage. A similar mechanic with the same idea (known as Alpha Counters) allows you to escape guardstun.
To code a basic combo breaker, add a move that can only be done when StateNo = 5001 or 5011. These are the default states for hitstun (standing and crouching respectively) after hitpause has expired.
;Light Kung Fu Knee
[State -1, Light Kung Fu Knee]
type = ChangeState
value = 1050
triggerall = roundstate < 3
triggerall = statetype != A ;cannot be in air
triggerall = command = "FF_a"
triggerall = power >= 1000 ;must have at least 1000 power
triggerall = !HitFall ;cannot be in a fall state
trigger1 = StateNo = 5001
trigger2 = StateNo = 5011