A unit is:
1. A set amount of money that one wagers on a game. For example, if you commonly bet $50 per game, then your unit is 50 bucks. If you find a game you really like and want to bet 2 units, then you'd bet 100.
2. A schlong, cod-piece, yambag, slang term for Jim and the twins also known as Ding-Dong, tallywacker, or Big Pete