There's a tooltip on the "Participation" button at the top, which you can see if you hover over it:
Users most active on this site in the last 60 days (combined number of posts, votes, comments and edits).
This post elaborates more on it:
Take the sum of those four values and sort the users based on those sums. The only minor details that are not in the tooltip are things like "don't count deleted stuff", and the fact that "votes" includes answer acceptance and that "edits" excludes edits to your own posts.
As site metas don't have reputation, the designers needed a different way to order users who participate on them, so this is what they came up with.
Incidentally, this sort order is also available on main sites despite there not being a link to it; it can be accessed by going to