Overview
A big bet (BB) is the larger of two fixed bet amounts in a fixed-limit poker game. A big bet is used in the final rounds of a game to increase the pot amount and thereby enable the possibility of a bluff. Big bets are generally double the wager of the initial or small bet. Any multi-round poker game can use big bets to standardize wagers while maintaining a sufficient risk-ratio to encourage bluffing. Casino poker tables use big bets to set a limit to the amount of money a patron can lose in each wager.
Statistical analysis
Big bets are used in place of variable limit raises to add considerable risk to staying in a game until a hand is shown. This added risk enables other players to bluff or to win a considerable pot when proving that they weren't bluffing. Other methods of adding structure to poker games include buy-in limits and maximum raise limits. Some sort of table or bet limits are required in poker to keep a person with the "deepest pockets" from "buying the game."
Examples
While any multi-round poker game can use big bets, the unlimited buy-in nature of casino style play is best suited for BB limits. Casinos can advertise the relatively low maximum wager of the BB as a way of attracting players, and players can join the table at any time.
Casino-style draw poker
Big bets are used in draw poker during the final round of betting to weed out tentative players. In theory, only those committed to their hand after seeing their final cards will be motivated to wager twice as much as their previous bet. In practice, however, additional motivation for players to fold is usually needed in a single draw game such as: a half-pot limit, a pot limit, or a spread limit. No limit poker is only employed when table limits are imposed, thereby disallowing casino guests to join the table after play has started.
From Wikipedia (CC BY-SA 4.0).