In a more generic sense, a referral tracker is simply a way to count the number of views to your hubs from different sources. The original intent is as Paul has said, but it can be used for other purposes.
I mostly use referral trackers to see which of my backlinks and social media are bringing me hub views. I have about 30 trackers, though some are not in use (and there's no way to delete them, unfortunately).
For example, say I have created a tracker called "viewedfromfacebook". When I ask for a link to a hub that I intend to post on Facebook, I'd select that tracker from the link creation popup. Then I'd use the URL it created for my post to FB. Anytime someone passes in through that URL, it increments the counter.
This usage of referral trackers has helped me a lot to see which backlink sources are the most effective.