The referrer is a piece of URL data a user web page request .

According to Wikipedia , "The referrer field is an HTTP header field that identifies the address of the web page linking to the requested resource. By checking the referrer, the new web page can see where the request originated."

The HTTP Referrer request header

The referrer contains an absolute or partial address of the page making the request. When following a link, this would be the address of the page containing the link.

When making resource requests to another domain, this would be the address of the page using the resource. The Referer header allows servers to identify where people are visiting them from, which can then be used for analysis , logging, optimized caching, and much more.

The Referer header cannot contain URL fragments (i.e., “# section”) or “username:password” information query . What is sent, if anything, depends on the request's referral policy.11

Source: https://developer.mozilla.org/en-S/docs/Web/HTTP/Headers/Referer