The said code will give you the full url path to which your user came from.
Example is http://google.com/?some-strings=goes-here
So you can check if its from a user from a google with a particular search string.
Just a Note. It can also received the same domain to which your site is in. For an instance your domain is http://domain.com then you referesh the page, the said code will return http://domain.com because you just refresh the page.