Referral trackers codes used to create special links to hubs (articles) on hubpages.com. When someone clicks on one of these links and then signs up for an account with hubpages, the "owner" of the referral code gets credit for the signups. Then if the person who followed your referral link creates hubs of their own, you will get credit for a portion of the advertising revenue that they generate.
A good way to use referral trackers is to create links on other websites pointing to your hubs or even hubs written by other people. This way if you bring new users to hubpages you can benefit from any articles that they create. NOTE: the fact that you get a portion of the advertising revenue generated by the people who sign up using your referral tracker does not take away anything from the new signup. They will earn the same no matter how they joined hubpages. This is a win win situation.