What does it mean when a QB calls out Omaha ?

When a QB calls a play in the huddle, he usually gives a snap count aswell, something like "Slot Right, Y Jet, Smash, on 2". The "on 2" bit is the snap count, if he gets to the line of scrimmage and the play clock is low, he will call omaha, telling the linemen to forget the snap count and go on the next sound. Sometimes the QB will fake the omaha call to catch out the defence.