In a game like Texas Hold'em there is a small blind and a big blind. In some games the players agree to a straddle as well.
If you take the example of High Stakes Poker on GSN, all players ante $100 every hand. The small blind puts in $300, and the big blind puts in $600. If all the players at the table agree to a straddle, the player to the immediate left of the big blind puts in $1,200 (ie double the big blind). Any player wishing to remain in the hand has to at least call the straddle or raise.
The idea is it adds more action to the game and makes things more 'interesting.' Even the blinds have to commit a lot of money to the pot just to see the flop. Just like the blinds, the straddle is put into the pot before the cards are even dealt, so a straddle is like a third blind.

