A referrer (spelled "Referer" in the HTTP specification due to a historical misspelling) is the URL of the web page that contained the link a user clicked to arrive at the current page. The browser sends this information in the Referer HTTP header with each request, allowing the destination server to know where the traffic originated.
Referrer data is fundamental to web analytics. It reveals which websites, social media platforms, and search engines are driving traffic to a site. For URL shortening services, referrer information helps link owners understand the context in which their short URLs are being shared and clicked. Web analytics books on Amazon explain referrer analysis.
Referrer data has limitations. HTTPS-to-HTTP transitions strip the referrer by default, many browsers and privacy tools suppress or modify the referrer header, and direct navigation (typing a URL or using a bookmark) produces no referrer. The Referrer-Policy header allows site owners to control how much referrer information is shared.
For URL shortening services, the referrer of a short URL click indicates where the link was placed (a social media post, an email client, a website). This data complements UTM parameters and provides additional context for traffic analysis. Digital analytics books on Amazon discuss attribution models.