A referrer is the last page viewed before clicking over to a landing website or page. So, a referrer is a source of visit(s).
In accordance with Internet Protocol, the referrer page is transmitted to the destination website server by visitors’ browsers and identified by his URL which is called referrer URL.
Referrers analysis is an important part of web analytics and very useful to identify and analyze traffic sources.
Search engine referrer URLs are analyzed and processed to extract keywords used by visitors to access the website.
In the example shown below, the referrer URLs reveals the "what is referrer" query made on Google.com.
For referral sources, retrieving referrer URLs allow to identify referrer domains and page (backlinks) and to see on theses pages how the "recommendations" and anchor texts are made. It is important for reputation analysis and for backlinks studying with a SEO perspective.
Paradoxically, all referrer pages are not considered by web analytics tools as referral sources. Search engine referrer pages are classified as search sources.
Referrers among other traffic sources on Google Analytics: