HTTP referer (originally a misspelling of referrer) is an HTTP header field that identifies the address of the webpage (i.e. the URI or IRI) that linked to the resource being requested. By checking the referer, the new webpage can see where the request originated.
In the most common situation this means that when a user clicks a hyperlink in a web browser, the browser sends a request to the server holding the destination webpage. The request includes the referer field, which says the last page the user was on (the one where he/she clicked the link).
Referer logging is used to allow websites and web servers to identify where people are visiting them from, for promotional or statistical purposes.
Referer is sometimes used to combat cross-site request forgery, but such security mechanisms are limited by the ease of disabling or forging a referer. A dereferer is a means to strip the details of the referring website from a link request so that the target website cannot identify the page which was clicked on to originate a request.
Read more about HTTP Referer: Origin of The Term referer, Details, Referer Hiding