This statistic counts the number of times an intended receiver touches the ball but fails to catch it. If the ball is thrown but the receiver never gets his hands on it, it is not recorded as a drop. It is recorded by some teams and publications, but not officially by the NFL.
Dropping passes is an accepted part of the game, but professional receivers are expected to catch every ball thrown their way that is not batted down by a defender or outside their reach. As such, dropped passes are recorded to keep track of which receivers are actually catching the ball when they are supposed to. Typically and on average, a professional receiver will drop about 8-9 passes over the course of a season. The widely recognized leader of passes dropped for a season is Terrell Owens, but finding exact numbers is difficult.