From MozillaZine Knowledge Base
HTTP is the application-layer protocol that most web pages are transferred with. 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. The HTTP spec specifies that going from a secure (https) server to a non-secure (http) server should not result in a Referer header being sent, but does not define whether a Referer should be sent between two secure sites. This preferences controls that option.
Possible values and their effects
Send the Referer header when navigating from a https site to another https site. (Default)
Don't send the Referer header when navigating from a https site to another https site.
Those concerned with privacy can set this to false, 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