When a web page is requested from a server, an optional field known as the referrer is sent via the http//: header. The primary purpose is to reveal which page a specific user was on prior to arriving at the current page. This makes it possible to locate the "referring website" and follow a customer's path as they move from one website to another. By having this information, website owners can provide a more customized user experience and block specific users who appear to be visiting questionable websites.
Our ‘Top Referrer’ tool automatically detects where traffic was just before it arrived on your site. The domain names of these sites are shown as the referral traffic sources in your reports. You can create a filter to block traffic coming from certain unwanted domains. This method of excluding referrers prevents all hits coming from the websites you've excluded from contributing to your Analytics traffic. One justification for doing this is to stop legitimate hit data from being contaminated by spam hits.