To either having bet or call all of your current chips in a hand. Usually in a poker game, a player may not buy more chips than he already have during a hand. If you don't have enough chips to call a bet, you may go all in and bet your chips against the same amount of chips of your opponent, that if you win you get the amount of chips you have multiplied by the number of players (rest of the pot money goes to second best). After an all in bet, a side pot may be created, in which the all-in player won't be an active player.