Forward rotation on the ball created by brushing up the back of it with the racquet. Topspin makes the ball dip down faster and bounce higher, giving you bigger margins over the net while still keeping the ball in the court. Nadal's forehand generates absurd amounts of topspin, sometimes over 3,000 RPM. Modern tennis is built on topspin. The Western grip revolution made this the default.
"Heavy topspin forehand that kicks up to shoulder height after the bounce. His opponent is backed up three feet behind the baseline trying to handle it."