Here's a good (if somewhat lengthy) explanation courtesy of Page1 SEO tactics:
"To create a tracker on HubPages you will need to pick a tracker name that is unique because you are able to use these trackers on any hubber's hub when you share them on social networks (this allows you to earn money from views you bring to hubs that you are not the author of). Of course you'll want to be on the referral tracker page (for those who don' know where that is) by clicking my account > earnings > referral trackers (referral trackers is located on the left sidebar). Your default referral tracker and any other trackers you create will display at the top of the page, and below your trackers list is a form to create your own referral tracker, which is where you will enter your unique tracker name.
To create your referral tracker you will enter an underscore line ( _ ) along with the tracker into the URL of the hub or HubPages page you are sharing right after a forward slash placed after the HubPages.com domain. A demonstration would be best in order to translate that into plain English that is easy to understand, so I'll show you the URL to the HubPages sign up with and without a referral tracker."
Again, referral trackers are a way to receive a slice of the earnings for someone else's Hub that you promote, likely on social media. It's good for you and good for them, so definitely utilize it!