All In is a phrase commonly heard in
poker games, especially
Texas Hold 'Em, implying that the player is betting all his
chips. It is a phrase holy to almost all poker players, and the
urge to say it can be so great sometimes that you bet all your chips away just to hear yourself say those two
glorious words.
Hearing someone else say All In can be quite traumatic, unless of course you have a brilliant hand, because it puts you in the high pressure situation of deciding whether you want to match his bet and possibly lose a rather lot of chips, or do the safe move and fold, allowing the other player the satisfaction of taking the pot.
I have been burned by my share of All In's and I've taken home my share of huge pots from them. All in all (pun 100% intented) I'd say that going All In is probably the most entertaining move in poker and it may be the reason why so many people play the game.