On ( poker )

1) Up to; that is, referring to the person on whom the action has stalled. "Who's it on?" means "Whose turn is it?"
2) In agreement; usually about a bet, a proposition, or a drink pot. "Are we on?" might mean, "In reference to my suggestion that we both put all our chips in the pot and take four cards, will you go along with me in this sporting endeavor?"