Sweet Peak is a competitive league that is available after all regular levels in the game have been completed. You can recognize it in the game when a level number is not visible when starting a level on the main screen. 

When a Sweet Peak is unlocked, you can play a random assortment of levels. Passing a level will earn you a trophy. Players are ranked on the leaderboards based on the number of trophies earned. Sweet Peak Season runs for one week, and at the end, the top players with the most trophies will earn rewards.

The next Sweet Peak season will then begin, and you can unlock Sweet Peak again by completing all newly released levels.