The big blind is a forced bet made in some variants of poker by one player at the table. Its most common use is in hold 'em games. In most games of hold 'em, there is also a small blind to the right of the player with the big blind, but in some games, there may only be one (big) blind.
In limit games, the big blind amount is typically the amount of the small bet: thus the player who posts the big blind is considered to have "already bet" the full small bet for the initial preflop betting round. In no-limit games, the big blind amount is set by the definition of the game (e.g. a $200 max no-limit game with $2/$5 blinds has set a $5 big blind amount).
The big blind is the player two seats to the left of the player with the dealer button (or, only one seat to the left if the game has no small blind). Initial betting action in the preflop betting round begins to the left of the big blind and continues clockwise around the table, with the big blind acting last. If no other player has raised the bet, the big blind may choose to check (and close the betting round) or they may raise. In all other ways, the big blind is treated as though they had initially started the betting by placing their initial bet into the pot, but in this one way (the fact that they can raise when the action comes around to them again) they are treated as though they had not bet at all. Otherwise, the player would never have the option to raise.
The strategy for playing hands in limit hold 'em in the big blind position is much more loose than other positions. For one thing, in many passive low-limit games, the big blind will often be able to see the flop "for free" (without putting any more money in the pot), and this will of course occasionally lead to unusual flops like a Big Blind Special. Even if another player has raised, a player in the big blind is often getting remarkably high pot odds to call with even marginal hands, especially at passive low-limit games where five players may see the flop even in a raised pot: if four other players are already in for two bets each, the big blind is getting 9:1 on their money to call the last bet, and can play any pair (even deuces) profitably, and many mid-range and suited cards.
However, the player in the big blind must always be careful in post-flop play, since they will be badly out of position on every subsequent [betting round]].