The directions that a quarterback will say to the offense to determine when the center should snap the ball to him. A snap count is often changed from play to play to keep the defense from being able to anticipate the snap and get a faster start. Snap counts often can hide changes to offensive plays, as well as give directions to players regarding the alignment of the defense.
A snap count is a useful tool for a quarterback in keeping the defense off balance. A team that becomes too predictable with its snap count gives the defense a chance to take off immediately upon the snap of the ball; this gives an advantage to the pass rush and forces the offensive line to react quicker. To combat this, teams will change the snap count on almost every play.
A quarterback can also use a "hard count". This is a loud signal by the quarterback that doesn't represent the actual snap of the ball. In a best-case scenario, a hard count can draw the opposing team offside, resulting in either a penalty or a free play.