The Big Blind is the large end of the ante in Texas hold em. The amount of the "big blind" defines the betting structure in limit hold em.
In a game where the tables stakes are defined as $1 and $2 the big blind most post the $2.00 initial bet before having a chance to see any cards. The purpose is to keep the action moving and force players to play actively.
The Player seated just to the left of the big blind has the option of calling the blind in which case he must bet the amount of the big blind. Assuming there have been no raises the betting comes around to the small blind who can then call of raise.
The big blind then has the option to bet as the last person on whom the action is put. He can raise and start a new round of betting