In tennis, everybody wants to hit a “heavy” ball. What does that even mean...?
Say you were trying to apply force on an object, there’s a difference between a “slap” and a “punch” even if your hand is moving with the same velocity. A slap clearly has no weight behind it, while a punch drives through the target at impact. In a serious fight, my guess is you'd rather be throwing punches.
In physics, they say force = mass x acceleration. A lot of coaches emphasize racquet head peed, but it’s not the actually pure swing speed that you’re looking for. Sometimes a slower swing might produce a heavier, more powerful shot. Hitting “heavy" involves you engaging your legs and core strength through the ball, giving the ball weight/mass that slapping at the ball does not.