The Leaderboard is a feature that you'll only see after point tracking has been enabled in Study Settings: Points. The Leaderboard is accessible from the Participant Directory icon menu (as shown below) and also has a dedicated tab in the directory itself.
The Leaderboard essentially lists Study participants from the most to the least points accumulated. The Leaderboard will also feature Levels which can be configured to showcase participants that reach a particular point range as shown below.
You can limit participants access to the Leaderboard in Study Settings: Points.