An referrer URL is a complete URL corresponding to a referrer page.
Search engine referrer URLs from SERP 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 classic referrer pages, the URL allow to track down the referrer and examine the backlink and his context.
See more on referrer definition.