Refers to the top person in a group, often with a nuance of being the central figure or ringleader, especially in informal or slightly rough contexts.
He is the boss of the local yakuza.
Who is the head of that group?
Compound of 親 (oya, 'parent') and 玉 (dama, 'ball' or 'jewel'), literally 'parent ball'. The term likely originated from the idea of a central or largest bead in a string of beads, metaphorically extended to mean the central figure or leader of a group.