Calling an audible is what happens when someone, usually the quarterback, changes the play at the line of scrimmage. Why? Usually for one of two reasons: 1. If he doesn’t think the original play called will be successful against the defensive he’s seeing, or 2. If he sees some weakness in the defense he thinks he can exploit with a different play.
Peyton Manning, king of all things offense, does this frequently. Here’s an example:
You’ll notice when an audible is being called because the quarterback will be in make-things-happen mode, waving his arms and yelling and pointing, and players will be switching places to line up for a newly called play.
So, in essence, calling an audible means changing the original play call to a different play that will give your team a better chance to succeed. Like when you audible to pizza for dinner instead of meatloaf as planned to please the masses on a Friday night. (Always a good play call.)