CNAMEs can only be set for subdomains that are registered with 1&1. It is recommended that you read the Explanation of CNAMEs first.
If using a CNAME, all DNS settings configured through the 1&1 Control Panel will be discarded. A CNAME can be thought of as a kind of a pointer for DNS. Instead of specifying DNS settings for a domain in the Control Panel, using a CNAME indicates that the DNS settings for the subdomain should be exactly the same as the DNS settings for the CNAME address.