RapidSSL is the certificate vendor, and their certificates are not yet fully trusted by at least some versions of Chrome.
(Notice now Chrome isn't on RapidSSL's list [icons at the bottom]):
So, this doesn't mean your connection isn't encrypted. It is. It doesn't even mean that anyone could decrypt it more easily than a SSL certificate from any other vendor. They can't. It just means that you have to tell Chrome to accept the certificate.