Cryosleep is a method used during space travel where the person is frozen into a deep sleep so as to pass the time of travel more quickly. This allows the person to not consume any food or rescources that would otherwise be needed if a person was awake. Riddick is heard saying in Pitch Black that "the brain shuts down in cryosleep, all but the primitive side," which he says is why he was awake unlike the other members of the ship he was on.
Cryosleep has also been used at some prisons as a means to keep the most dangerous prisoners out of circulation. In The Chronicles of Riddick: Escape from Butcher Bay, Hoxie, the warden of Butcher Bay, decides that since Riddick is being such a problem he is going to place Riddick in cryosleep. Riddick was subsequently always asleep except for 2 minutes a day of "exercise". It was during this period of time that Riddick made his escape.