The quarterback is an offensive position in the games of Canadian football and American football. A typical offensive play begins with the quarterback being handed or thrown the ball in a maneuver called a "snap" from the offensive player known as the "center". The quarterback may then throw or hand the ball to another player, or attempt to advance it himself. If he is tackled while behind the line of scrimmage, before he has a chance to do any of these things, it is known as a sack.
The quarterback is the offensive leader of a football team. Although he does not usually choose the team's offensive plays (generally the coaches choose the plays), he usually tells the play to his teammates in the huddle. The quarterback also often gets to choose the "count" of the cadence for the snap. The quarterback usually calls out the signal for the ball to be snapped and may optionally change the play at the line of scrimmage before the ball is snapped by shouting out additional signals to the other players on his team. This is known as calling an audible. A typical cadence might be something like, "Down, Set, Hut", where the ball is snapped on "Hut". If an audible were to be called, it would be shouted somewhere between "Down" and "Hut". For strategy, the ball may be snapped on "Set" or after multiple shouts of "Hut" to catch the defensive team off guard. This is what is meant by the count of the cadence.