Martial Adept is a passive feature provided by the Feat of the same name. You learn two manoeuvres and gain a Superiority Die to use them.
Description
Learn two manoeuvres from the Battle Master Fighter subclass and receive 1 (additional) 
Superiority Die to fuel them. You regain expended Superiority Dice after a Short or Long Rest.
How to learn
Granted by the feats: Martial Adept (Feat)