Tachyons were particles which could only travel faster than light, allowing them to travel in time.
Tachyons were produced whenever a parallel universe was produced. The parallel universe could have greater gravity, slower light speed, etc.). At the beginning of time, great amounts of tachyon radiation was created, since reality was very fluid. As time went on, the potential changes to the universe became smaller and more negligible and tachyons became rarer and rarer. (PROSE: Trace Memory)
Tachyons could be used to allow a being to travel in time (PROSE: Trace Memory), to age or de-age (TV: The Leisure Hive). The Tachyons could affect the universe before the events that caused them occur, since they would travel to the past. (PROSE: Trace Memory)
The Argolins attempted to develop tachyon-based regeneration for the Leisure Hive. The Tachyon Recreation Generator was unsuccessful until it was helped by the Fourth Doctor. Pangol used tachyon technology to clone himself, bringing copies of himself from the future. Eventually, the process was reversed and he was turned into a child. (TV: The Leisure Hive)