Idols are golden masks found randomly in the topside world. When placed on the hotbar and used, they summon an Oobaboo Chief that will give you one of his tribesmen as a follower for about two minutes. You can choose either a Gatherer, a Warrior, or a Healer.
A gatherer will mine items for you (With his bare hands!) Including... err... Bushes?
A warrior will most likley fight monsters and a healer will most likeley... heal. (Duh.)