To bet is to gamble or risk something valuable against an unknown outcome. Poker players bet on each hand, hoping the cards they hold will beat those of the other players.
Sometimes you bet money on a sporting event, wagering on the team you think will win and collecting even more money if it does. It's even more common to bet in a figurative way, saying something like, "I'd be willing to bet that the bus will be late again." The money you risk when you bet is also called a bet. The word probably comes from the now-obsolete noun abet, or "abetment."