Unconscious is one of the negative status effects in Pillars of Eternity. A character becomes unconscious when his or her endurance drops down to 0 during combat, but his health is still not 0 (that would lead to maimed or dead state instead).
If a character loses all endurance during a fight they are knocked out for the duration of combat, unable to move or take any actions, but also untargettable by any enemy attacks. Unconscious characters cannot benefit from endurance healing, but they can be revived to life via certain abilities. Once the fight ends they will regain consciousness and eventually recover their endurance (up to their maximum endurance or actual health, determined by the lower one of those two).