Information passed in the HTTP Header identifying the URL of the webpage that linked to the requested resource.
For example, if I visit the page http://somedomain.com/referrer.html and click on a link to http://anotherdomain.com/requested.html, the logs (and analytics) will show somedomain.com/referrer.html as a referrer to anotherdomain.com/requested.html.
This information is valuable for SEO purposes as it allows site owners to identify the sources of their traffic.