This error means that the hostname you are trying to connect to cannot be resolved to an IP address. (Hostnames are resolved to IP addresses by a DNS (Domain NameServer)).
Please check what you have entered in the Address field. You will need the valid hostname of an FTP server or a valid IP address. If you are connecting through the favorites, only the name or address of the FTP server belongs in the host section and you should not include URL or path information in this field.
If you have verified that the hostname is correct, there may be a problem with your DNS server (or with your ISP's DNS server, if you are a standalone user). It may be unable to resolve hostnames to IP addresses. At this point, it would be a good idea to contact your ISP about the problem.