The message means that the site to which you are trying to access will not grant permission to the URL making the request. Some sites do not like users to access certain web pages without first having to navigate through their site.
For example, if you try to access the following URL address:
you will receive the 403 message because you are trying to get into the site several directories deep. The server wants you to enter the site at w.checkupdown.com
explains it far better than I can. My hunch is that this is not a resolvable issue by us.