In Football for Dummies Howie Long (a former professional player) and John Czarnecki (Fox Sports commentator) write:
„In American football, the quarterback relays to his teammates in the huddle what play the coach has called. The play is a mental blueprint or diagram for every player on the field.
Quarterbacks are also allowed to audibilize, or change the play at the line of scrimmage. A changed play is called an audible. Quarterbacks usually audibilize when they discover that the defense has guessed correctly and is properly aligned to stop the play.
Everything the quarterback says in the huddle refers specifically to the assignments of his receivers, running backs, offensive linemen, and center.“
audible: heard or able to be heard.