Referral in Google Analytics refers to referral traffic. Referral traffic is used to describe visitors to your site who come from direct links on other websites rather than directly or from searches. For example, this happens when a site is mentioned on other sites, such as a recommendation. Referral traffic can also be created by leaving links on other blogs or forums.
Referral traffic is the method Google employs to report visits that came to a site from sources outside of its search engine. When someone clicks on a hyperlink to go to a new page on a different website, Analytics tracks the click as a referral visit to the second site. The source site is called a “referrer” because it refers to traffic from one place to another.