From BR Bullpen
A foul tip is a foul ball whose trajectory is barely affected by a glancing touch from the batter's bat. The rules pertaining to foul tips vary somewhat from those for other foul balls: a foul tip caught on the fly by the catcher is treated as a regular pitch that has been swung at and missed; the batter is not out unless the pitch constitutes the third strike. It is also legal to steal a base on a foul tip that is caught on the fly. However, if the foul tip touches the ground, it becomes a regular foul ball: the ball is dead and baserunners must return to their original bases.
A foul tip becomes a foul ball when it is deviated higher than the batter's head, or outside the reach of the catcher to one side or another of home plate.