From the Super Mario Wiki
Shooting-Star Save is a defensive Power Shot that Clay and Ace can use in Mario Tennis: Power Tour. It can be unlocked by obtaining 10 Response, 10 Jump, and 9 ESP from the player's PS stats. When used, meteors rain down on the user's side of the court, allowing them to arc the ball back to the receiver.
"Ride a shooting star down from the sky to lob the ball back."