The leaderboard is based on a dynamic score that's influenced by each user's recent activity, the quality of their database, and overall engagement.
To access it, go to the Dashboard from the left sidebar and scroll to the Leaderboard section.
Here, you can hover over a user to see key score drivers - factors highlighted in green contribute to the score, and factors in gray indicate areas of improvement that can be addressed to improve the score. 
Note: The score can sometimes take up to a day to reflect changes.