From MozillaZine Knowledge Base
HTTP is the application-layer protocol with which most web pages are transferred. As part of HTTP, requests can include a "Referer" (sic) header that tells the server which page the user was on that initiated the request. Servers use this information to track users' paths through the site and possibly provide additional features.
This preference controls when to send the Referer header and set
Possible values and their effects
Never send the Referer header or set
Send the Referer header when clicking on a link, and set
Send the Referer header when clicking on a link or loading an image, and set
Those concerned with privacy can set this to 0, realizing that this may adversely affect some sites. Those wanting to ensure compatibility should leave it at the default.
First checked in
Has an effect in