When the quarterback steps up to the center and prepares to start the play, he has a lot of work to do. His first job is to look at which defensive players are on the field and where they are, and try to deduce what the defense is going to try to do. He may see signs that the defense is lined up perfectly to stop his play. In this case, the quarterback will call out an "audible." He will shout out code words that tell the other players on his team that he is changing the play. Most often there is a special code word, chosen that morning, to tell the players that the play is changing. Perhaps the code word will be "blue," so to change the play the quarterback might shout out "Blue 23, Blue 23, Blue 23." If the quarterback is not changing the play, he still wants to confuse the defense, so he might shout out "Green 17, Green 17, Green 17." The other offensive players hear "green" and know to ignore this, but the defense doesn't know the "hot" color and has to wonder.....i took this straight from the site.