This is a phrase used in football which describes a quarterback calling a play or making an audible very quickly in an attempt to confuse or otherwise fluster the defense. A quarterback will tell their teammates in the huddle that they will be doing a quick count and provide instructions as to when the ball will be snapped. Most teams have many different ways of executing a quick count so the defense cannot catch on.
If a quick count is successful, it may result in a big play for the offense, or a penalty on the defense. Teams use the quick count when they believe they have an advantage over the defense. It is also used fairly often in fourth down situations where the team needs few yards to reach a first down. In this case, the quarterback may be able to get a play off before the defense is ready, or even draw them offside if they are trying to anticipate the play. A slow count is the opposite, and is sometimes used to accomplish similar goals.