The Great Ones were the gods of the People of the Trees. They were represented by three wooden statues. The Fourth Doctor believed that there was some kind of connection between the People and the statues.
Aulian Thorolis hired Zusala to steal the statues, but one of them was left behind. The psychic connection between the People and the statues was lessened, with two statues missing, and many of the People went into comas. After the Doctor and Leela defeated Aulian and returned the statues, the People came out of their comas. The leader of the People, Caramir, believed this event was a test of their faith. (PROSE: People of the Trees)