What is a referer?

When one website links users to another website, the first website refers the user to the second. Typically, this information is captured in the HTTP referer field in an HTTP header. (The term “HTTP referer” was originally a misspelling, but it has since been adopted into the HTTP specification.)

For secondary elements on a website, like images or advertisements, the referer is typically the HTML page that calls those secondary elements.

When you create exceptions that permit one or more sites, you can either permit those sites regardless of how they are accessed, or permit them only when they are linked from a site that you specify (the referer).

If, for example, you permit example.com only when referred by mycompany.com, users can click the example.com link on the mycompany.com website and get to the page. Secondary elements on the page, however, like images and advertisements, will not appear, because the referer for those elements is example.com.