The big bling is a position at the table and a mandatory bet. The big blind position is located two seats from the button directly to the left of the small blind, and the player sitting in this seat has to place the BB before even seeing his hole cards.
The size of the big blind is determined by the format you are playing. It is fixed in cash games and constantly changes and increases in tournaments.
Since the dealer button moves every hand, every player at the table will play at the big blind position once per orbit. Combined with the small blind, the big blind drives the action at the table by forcing players to have money in the game before the hand is even dealt.
For example, if you are playing in a $100/$200 Texas Holdem game, the small blind will be $100, and the big blind will be $200. Before even dealing the cards, there will be $300 in the pot so that players would be encouraged to fight for it.