A CNAME record is a DNS record which points a name to another name, instead of directly to an IP address like an A record does. Note you cannot use a CNAME to point to a sub directory, specific file or URL but only to a domain or a sub-domain. eg domain.com, www.domain.com, sub.domain.com is FINE; domain.com/mypage.html, domain.com/subdirectory/page.html is NOT.
For example lets say you want to create a CNAME for www.mydomain.com so that it points to www.anotherdomain.com
Then in the DNS Management section, within the CNAME creation area:
In the Subdomain field you would enter: www
In the Hostname field you would enter: www.anotherdomain.com. <— note the full stop terminator. If you did not put the full stop terminator there then you would end up with something like www.anotherdomain.com.mydomain.com