From the Super Mario Wiki
The Tournament Green is a course feature that appears in all Mario Golf games since Mario Golf: Toadstool Tour. They are similar to regular greens, but the ball will slide farther when it lands on the Tournament Greens, making it more difficult to make a good shot. Spin will also have more effect on these greens.
They are originally found in the Star Tournament as an additional challenge, but they can be used in any course once the player beats that course in the Star Tournament. In Mario Golf: World Tour, they only appear in large scale online tournaments, such as the Castle Tournament.