A sleeper was an astronaut from the Andromeda constellation. Because of the vast distances from the constellation to Earth, they needed suspended animation to make the journey. Hence, they were informally called "sleepers".
Three such astronauts found a way into the Matrix, five hundred years before the Sixth Doctor arrived on Ravolox. When they fled to Earth, the Time Lords devastated it with a fireball, transforming Earth into Ravolox.
The three escaped the Time Lord's wrath by building and hiding in an underground bunker. They also created the L3 robot later known as Drathro to maintain the bunker while the three sleepers put themselves in suspended animation. Just before the Doctor's arrival, the system began to break down. Drathro could not correct it because he was trained in installation and maintenance. As a result, the three sleepers died whilst in suspended animation. (TV: The Mysterious Planet)