In CPanel go to Domains -> Subdomains.
This will take you to the CPanel subdomain page where you can add and manage subdomains.
A domain must be added first using CPanel Aliases feature before a Subdomain can be created on that domain.
The sub domain is the part before the domain name separated by a dot. Example: mysubdomain.mydomain.com
It connects to it's own separate folder/content.