What should be specified in the 'Referrer' field?

In this field you can specify referrer - the website from which the robot will get to an appropriate page of your website from field 'Website page'.

As a referrer it is recommended to specify the websites that contain a link to your website. Setting 'empty' referrers, without such links, will make statistics unnatural that might be suspicious. If there are no links to your website on a referrer website, a robot will follow a link anyway, and the website you specified will be a referrer to your website.

You should remember that the website-referrer must be also available for a robot (it must not have a regional or any other limitation of IP addresses), otherwise the system will not be able to visit your website from it.