if you don't know, an audible is a change of the original play that they called in the huddle. The receiver will know what "omaha" means or "blue" means if the QB yells it out. Maybe the QB originally called a play for #88 but sees that #88 is going to be double covered when they are about to snap the ball. He will then call out "omaha" or "red" or "blue" or whatever their audible calls are and change the play at the line. All the players know what to do then because they are familiar with the audible calls.