A Critical Hit is a term for an attack that staggers the opponent (this is referred to as a Critical Stun), instead of knocking them to the ground. This allows for a series of follow-up of attacks, although all strikes will do 50% less damage. Staggered foes can still use holds (unless hit by what is referred to as a "forced sit-down"), but damage from holds during the critical state are reduced. Any strikes to a foe undergoing Critical Stun state is referred to as a Critical Combo. This is commonly used when performing combos, especially when used with Critical Bursts.
In Dead or Alive 5 and it's updates, when an opponent enters Critical Stun, a portion of their health bar flashes white to indicate the amount of damage that can be done to them before the Critical Stun is broken (i.e. doing more damage will knock the opponent down). This amount is usually determined by how damaging the initial Critical Hit was; generally, a more damaging Critical Hit turns a larger portion of the opponent's health bar white and thus allows for a more damaging overall combo.