Definition from Wikipedia:
Liar’s poker is a bar game that combines statistical reasoning with bluffing, and is played with the eight-digit serial number on a dollar bill. Normally the game is played with a stack of random bills obtained from the cash register.
The object is to make the highest bid of a number that does not exceed the combined total held by all the players. The numbers are usually ranked in the following order: 2,3,4,5,6,7,8,9,0(10)and 1(Ace). If the first player bids three 6’s, he is predicting there are at least three 6’s among all the players, including himself. The next player can bid a higher number at that level (three 7’s), any number at a higher level (four 5’s) or challenge. The end of the game is reached when a player makes a bid that is challenged all around. If the bid is successful, he wins a dollar from each of the other players, but if the bid is unsuccessful, he loses a dollar to each of the other players.