Insert the referer, if available: The Referer field allows the client to specify, for the server's benefit, the address (URI) of the document (or element within the document) from which the Request-URI was obtained.

This tag can be used for making a 'back button', that's a link back to the page he just was on (I use it for the [BACK] button in this documentation). It's recommended to use it together with <if referer>.