In the American game of
football, an
audible is when the
quarterback changes the play at the
line of scrimmage. This happens when the quarterback sees that the play called in the
huddle is not going to work against the defensive alignment, or that a different play has a much better chance of succeeding.
Generally, before the start of each play the quarterback can be heard calling out signals. If you listen closely you'll hear him yelling out something like, "BLUE TWENTY-SIX, BLUE TWENTY-SIX." Most of the time this is a "dummy" signal - or meaningless. But if "BLUE" is the audible code, then the quarterback is changing the play. He's yelling it out so that all the players on his team know the play has changed. And "TWENTY-SIX" is now the new play.
A good quarterback has to be able to read defenses and audible to the correct play. Good defenses will disguise their alignments to confuse quarterbacks.