Quarterback
If quarterbacks don't like the formation the defense is using, they may use an audible. An audible is a way for the quarterback to change the play at the line of scrimmage (where players line up to start a play). He changes what the play will be by shouting out things that his teammates will understand. An audible may be called if the quarterback sees how the defensive players are lined up and does not think that the play that has been called would be good against how the defense is lined up.