Colloquially, a player who hits the ball solidly is said to have gotten the "sweet spot" of the bat on the ball. The sweet spot classification quantifies that as a batted-ball event with a launch angle ranging from 8 to 32 degrees.
A player's sweet spot percentage -- or how often he produces a batted-ball event in the launch angle sweet-spot zone of 8-32 degrees -- is presented on Statcast leaderboards under SwSp%.
Why it's useful
Sweet spot percentage can be used in concert with hard-hit rate -- the percentage of a player's batted balls that have an exit velocity of 95 mph or higher. For a batted-ball classification that takes into account both launch angle and exit velocity, check out barrels.