From the Super Mario Wiki
The Tournament Green is a course feature that appears in all Mario Golf games since Mario Golf: Toadstool Tour. These kinds of greens produce less friction on the ball, making it roll farther than when it lands on regular 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 Expert Tournaments, which are played without a flight path.