Time crystals are rare minerals capable of manipulating time, guarded by Klingon monks on Boreth and used to power time travel.
The Klingon monastery on Boreth was guarded by monks whose order kept time crystals safe. In 2257 Captain Christopher Pike went there seeking a crystal needed to power a new Red Angel suit.
Touching it, Pike saw a vision of his own future, in which he was terribly injured while saving cadets. He took the crystal anyway, accepting that fate, and it enabled Michael Burnham to carry Discovery into the future.