What is an audible?  Usually, the quarterback will call an offensive play in the huddle.  But when he gets to the line of scrimmage, he might decide to change the play because of the way the defense is lined up--this is called an audible. 

For example, if the defense has a bunch of players lined up "in the box" (the space directly opposite the offensive linemen), this means that they are planning to defend against a running play.  Then the quarterback might want to call an audible for a pass play because there will be fewer defenders on the field focused on protecting against throws.

Former QB Peyton Manning was really well known for calling brilliant audibles. He had amazing recall of defensive formations that he had seen from watching tape for years.  If he recognized the defensive play at the line of scrimmage, he might yell to his teammates to change the play, i.e., an audible.  His adjustments would take advantage of the weaknesses in the defense's play call.

Play calls in football can have pretty amusing names. Check out this example, where Manning called an audible for a play nicknamed: "Bags Montana Fat Man."