In bingo chat, the letter "B" is often used as shorthand for "good luck" or "hope you win". It is a common way for players to show support and encouragement to each other during the game. For example, a player may type "B, hope you win!" or "BGL" (good luck) in the chat box. Using "B" has become a tradition in the bingo community and is a fun way to connect with other players.