In poker tournaments with an ante, instead of each player posting an ante each hand, the player in the Big Blind will post an amount equal to the Big Blind. For example: with blinds at 500-1,000, the player in the Big Blind will post 1,000 for his/her Big Blind, followed by 1,000 for the Ante.
Blinds bring less money in the pot than Ante, but they bring in enough money to keep some player motivated to bet. Especially, the players who place the Blinds stay in the game, as they have already put some money in.