This is enabled by default. When enabled, the existence of a referrer (referrer) header is checked for. Some spambots do not send this header, so would fail at this point. Enter your website domain below, to be used in conjunction with this check. If you enter your domain, then not only is the existence of the referrer header checked for, but it is checked to see if it contains your domain. It gives you an extra layer of protection against spam.