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.
Learn two manoeuvres from the Battle Master Fighter archetype and receive 1 (additional) Superiority Die to fuel them. You regain expended Superiority Dice after a Short or Long Rest.
Granted by the Feats:
Martial Adept (Feat)