Clubs have different properties: Power, Accuracy, Top&Back Spin, Side Spin and Guide Line. Upgrading the Club enhances its properties. 

To check your Club's stats, tap the Club Bag icon on the main screen.

If you own the Club, you will see the stats by clicking on it, and then the "UPGRADE" or "INFO" button.

If you don't own the Club, you can see the properties by clicking on it. 

For more information on the meaning of the properties, check out What do Guide Line, Accuracy, and Power mean?