The longer you stay a subscriber, the higher your level becomes! There are three levels: Master (3 months), Hero (6 months), and Legendary (12 months). At each level you get a certificate and an extra item, with legendary getting you an extra item every month permanently for as long as you remain a subscriber.
You don’t need to do anything to gain levels except stay a subscriber. Note that canceling your subscription will result in an immediate loss of your levels and a level reset, even if you attained a level that month.