Referer is the HTTP request header field that contains the address of the webpage from where the request for a web resource originated.
Referer tells the web server the address of web page from where the resource was requested. It is a misspelling of referrer but the incorrect spelling is used by most of the standards. Referers are generally logged by web applications for analytics. Referer is not present for HTTP requests where the link is entered manually in the browser or it was bookmarked. HTTP referer can be easily spoofed/blocked by the client.