Beats me. It could be a feature that's disabled because I can't find anything that would constitute a referral. Traditionally, it would be a count of people who joined after following an invitation link provided (referred) by you.
I just logged out and tried to register as a new user. There is a "Referred by" box where a new member can enter an existing member's name who told them about the site. Apparently HoosierHiver talks us up quite a bit and it's working.