We've put together different 'leagues,' which essentially breaks up the leaderboard. We want players of all levels to find value in this site, and with one leaderboard it is kind of overwhelming to start at the very bottom. Having leagues gives you some milestones to work towards.
Different leagues also means we can have different requirements for each league. As time goes on and more replays are uploaded, we will likely make more games required in order to be listed on the top leagues. We'll always keep the entry requirement into the bottom leagues lower, so newer players can hopefully see themselves on the list pretty quick.
Leagues are just splitting up players by MMR into groups. After excluding players with less than 10 games played, Master League is the top 1%, Diamond is the next 9%, Platinum / Gold / Silver are each 20%, and Bronze is the final 30% of players.
Can't find yourself on the leaderboard? Your MMR may place you in a higher league, but you don't have the required number of games to be eligible. The MMR markers for each league are calculated before checking if players have the required number of games.