An Exhibition in tennis is a match played for fun or charity, not for official rankings.
Players compete to entertain fans or raise money for good causes. These matches don't count for ATP or WTA tour points.
Top players and even recently retired players often take part in exhibitions. In exhibitions, active players can try new things and show off their skills without pressure.
Exhibitions sometimes have special rules or formats to make them more exciting. Fans get to see their favorite players in a relaxed setting.