The wakeup kick is an action in nearly all 3D fighting games that allows one to kick the opponent while one is knocked down.
In most 3D games, unlike most 2D games, players who are knocked down still have a damage hitbox, so moves with a low strike height at some point in their arc have the opportunity to hit the person on the ground. Therefore, one’s actions on wakeup are extremely vital to success.
For this reason, a wakeup kick is useful for warding off overeagerly approaching opponents, but in most cases, they will put you at a disadvantage if blocked, so it is not something to abuse in the least.
In nearly all games, there will be two classes of wakeup kick: overhead and low (obviously, if getting blocked puts you at a disadvantage, the opponent should not be able to predict the right height 100% of the time).