If a batsman is injured or taken ill during his innings and forced to leave the field of play, his place will be taken by the next man in and he will be recorded as Retired - not out. If he recovers later on he may resume his innings at the fall of a wicket.
Sometimes, particularly in friendly, social matches, a batsman will retire because he wants to give someone else a chance to bat. He will then be recorded as Retired - out. In these circumstances he will not be allowed to resume his innings without the permission of the fielding captain - who is not obliged to give it.