enigma1 - 12:29 pm on Apr 22, 2011 (gmt 0)
It means there is no rDNS for the ip. In other words there is no domain name associated with a given ip - there is no ptr record. When you visit a site there is an IP associated with the site. The rDNS is the opposite.
It should not be used as a general criteria to restrict access to a site if that's what you're asking. But it could be used for other cases like when receiving emails or if you are certain there is rDNS (for instance popular search engines typically have one when they visit your site).
Also performing rDNS on its own doesn't do much. An IP could resolve - for example to localhost or any domain name just because it's mis-configured by error or on purpose. Therefore after you retrieve the PTR you would have to do additional testing to ensure the PTR resolves now to the original IP.
Also this kind of detection methods should not be used on the long run.