A sub-domain is simply an extension of your existing domain name. There is no cost to create a subdomain name. Your main domain name can be for example: vnkb.com. For a subdomain you can enter your control panel and create: james.vnkb.com.
This would be an example of a subdomain. A subdomain is an additional web address that you can create that is connected to your primary domain name. If your main domain name is: vnkb.com
Other subdomains you can create: phil.vnkb.com, roger.vnkb.com, albert.vnkb.com, etc.