Consider this Chemistry proposal. Clicking on the "most active" tab gives you a list of people who were, as the hover tooltip says, "most active on the beta site".
Now, I am not entirely sure of what all activities of the users count as "active". I am also not sure of exactly which metrics does Area 51 track from the activity of each user, and then is able to sort them into a list.
Of course, I can guess that answering and asking questions, editing posts and flagging spam are the most active things a person can do. But, a person can also upvote, downvote, and comment. Besides, I do not know the relative weightage each metric gets over the other in determining the "total activity".
I hope to get some official information regarding this.
I am new to Area 51 so please comment for clarification. Thank you!