After your opponent hits the ball and it bounces on your side of the table, look for the ball to glow green. When it glows, it means it's in the perfect zone. Hitting the ball in the perfect zone means you get maximum power or spin on your shot. You can also only hit smashes or Pro Shots if hit in the perfect zone.
If you return the ball outside of the perfect zone, you'll get reduced power or spin on your shot.