It's not RSS, since a traditional RSS feed link appears beside it, and clicking on it seems to increment the number of subscriber by one, but other than that scouring over the Stack Overflow UI does not appear to reveal anything. Surely this feature would have more use than serving as a popularity contest for the tags?
Apparently it allows you to quickly set up email notifications for a given tag.
Also, it gives a reason to give premium visibility to tag wikis, which is great.
I don't think the subscriber count should be given that much importance; a question count would have more appropriate. Alas, that's a matter for another feature-request.