"What exactly is a CNAME?"
CNAME stands for 'canonical name record' which is an overly fancy name for a domain alias. Domain aliases are often used to create custom domain names in order to maintain consistency between a desktop site's domain name, and a mobile site's domain name.
Ex: A demo mobile web site's URL might be: http://demosite.prohost.mobi. The companion desktop site's URL however might be: http://www.demosite.com. A CNAME would allow one to standardize the URLs and set up the mobile URL as: http://m.demosite.com
Note that the 'm.' subdomain is commonly used to designate a mobile website, but it is not mandatory. Any subdomain can be used (e.g. mobile, mobi) - the only requirement is that the chosen subdomain must be configured on both the MoFuse platform and within the CNAME record created in desktop site's DNS configuration.