1) Put in the maximum number of raises in a round of betting; usually followed by the bet, the bets, or the betting. Make the maximum raise permitted in the current round. "I'll cap it" means that someone has put in the, say, third raise.
2) After dealing the first round in a draw game, put a chip on top of the undealt cards for protection; usually followed by the deck.
3) The point at which the rules dictate that the maximum bets have been made. "The cap is lifted heads up."