It was a portable device, worn with a belt. It let its bearer to "time jump".
Fenris the Hellbringer employed one to travel back to the early days of Gallifreyan history, charged with averting the dawn of the Time Lord civilisation. His attempt was foiled by Rassilon, who eventually took from the time-jumper the directional control device, a piece of technology still missing from the Gallifreyan time travel technology. (COMIC: Star Death)