Referer log
When a client clicks a link to other.example.com on the website example.com, then the website example.com is a referrer and the client is referred to the website other.example.com. When a client is referred by a website, a HTTP header referer is sent by the HTTP clients. Squid has the ability to log referer HTTP headers, which can later be used for analyzing traffic patterns.
Note
"Referer" is actually a misspelling of the word "Referrer", but it has been officially specified that way in HTTP RFCs.