For those who don't know:
http-referer is an header, sent by the browser, which tells a page what was the address of the previously visited page

for example, if you visit endchan, and then you visit 4chan, the 4chan site will know you were previously on endchan, this applies mostly only to links, pages opened by a clicked link carry with them the http-referer of the site the link was from

the http-referer is often used in two main cases:
1) by site owners to track which sites are linking towards their site, this information can be used to direct advertisement of the site
or if the receiving site is malicious, to target the linked sites, with raids or glownigger takeovers
2) by server owners who want to block their content from being accesses by direct linking, by forcing their content to be loadable only if they carry an http-referer from the site itself, or from a limited list of approved sites

in few other cases the http-referer is used in forms with buttons, either login forms or other request forms (i guess the form method POST is based on the http-referer, if i'm not mistaken)