This is a baseball term which describes a pitch thrown far inside, forcing the batter to fall to the ground in order to avoid it. A pitcher may throw this type of pitch on purpose for many reasons, such as to move the batter away from the plate if he or she is crowding over it, to shock the batter, or even as a show of skill, to let the batter know the pitcher has good ball control. Though obviously discouraged, this type of pitch may also be used as a warning sign or outright revenge by one team for a perceived or actual slight by the other team.
If a pitcher throws this type of pitch, he/she is also said to have "dusted-off" the batter. There may be times when the pitcher throws a dust-off pitch by accident, but most Major League pitchers have good enough ball control that this does not happen often. A dust-off pitch is a dangerous pitch to throw because if thrown too far inside, it could actually hit and injure the batter.