Every leaderboard needs a list of players. The Player Source tells Leaderboarded where you want to get your list of players from.
There are 6 possible sources:
a. Manually adding players one by one
b. A Google Spreadsheet
c. Custom / API – for developers
d. Twitter – based on a twitter keyword
e. Leaderboarded – from other leaderboards
f. Nimble – from your CRM system