This is a poker term used to descibe a game that allows the players to bet or raise the entire pot when it is their turn to act. The rule when someone bets in front of you is that you can raise the pot plus the amount it would cost you to call the previous bet.
The wagering in this game increases exponentially, which means that your entire bankroll can be at stake in almost every hand. Sometimes you can also find a game that is half pot, so that only 1/2 of the pot (plus any raises) can be wagered.
This type of poker is contrasted with limit poker which has fixed betting amounts (either fixed limit such as $3-$6 where all betting is in those increments -- or spread limit such as $2-$10 where the betting can be any amount between the limits). The game is also contrasted with No Limit on the other extreme in that your entire bankroll can be bet at any time without limit.
They play Pot Limit Texas Hold-Em at The World Series of Poker, but it's not the main event, which is No Limit Texas Hold-em.