Bidding is an auction mechanic common to a variety of board and card games where two or more players or teams compete for a right or privilege. Bidding is common in trick-taking card game such as bridge, and several other bidding examples are provided in the bidding wiki page.
In Bridge, a bid is a proposal to make one or more tricks above a minority "book" of six tricks, in a "trump" suit of choice, or with "no trump." The highest numerical bid buys the contract. Between two bids that are equal numerically, the suits are accepted in a descending order of no trump, spades, hearts, diamonds, and clubs. It is preferable, when asking such a question, to specify the system used (such as Standard American or Acol).
In Spades, each team of two players bids for the right to take tricks during the proceeding hand, with the highest bid winning the honor. The team who loses the bid attempts to set the winning team by preventing them from making their bid.
In Axis and Allies, bidding is a popular rule variant that attempts to even out what is commonly considered an un-balanced game by spotting the weaker Axis additional units (equal to the value of the winning bid) at game start.