A blind bet, or blind, is a forced bet that must be posted before you see any cards in a game of poker. Blinds are an alternative to antes for getting money in the pot. Blinds are most often used in flop games like Texas Hold-Em and Omaha.
Typically the two players to the left of the dealer button are forced to place blind bets. The small blind (to the dealer's left) is typically half the size of a small bet, and the big blind (to the small blind's left) is a full small bet. Betting starts with the player to the left of the big blind (who is considered "under the gun").