A Stagger is a hit or event that causes the target to stumble or recoil backwards, forcing them to spend a few seconds to right themselves before they can act again.
A staggered creature loses the next frame(s) it would have during combat, either for actions or reactions depending on the current turn. For example, if you're staggered for one frame while it's not your turn and you haven't reacted yet, you lose your reaction frame. If you have reacted already, then you lose the first frame of your next turn.