The browser stores some information about HTTP request headers. They're useful for many purposes. One of the headers is the referral information. It's the previous referring URL of the current URL. URL Referrer Concept This post has links to another posts. If we go to any of those (click the link), then this URL will be the referrer of that link. Similar to ordinary referral (referring) concept in daily life, but within web (HTTP) realm. Legendary Misspelling Year Context Spelling Used Notes Pre-1996 (until now) Standard English Referrer ✅ The correct spelling in proper English. 1996 HTTP/1.0 Specification (RFC 1945) Referer ❌ A typo in the RFC that became part of the official HTTP header. It was never corrected to preserve backwards compatibility. Late 1990s - Early 2000s ...