A sudbdomain is a domain that is part of a larger domain.
To add a subdomain in cPanel you need to first login to your cPanel account.
Under the Domains section you will see a button titled, “Subdomains”. To add the subdomain you need to click on this button (See pic below, click on image for larger version ).
Once you click on the Subdomains button, you will be presented with a new window for adding the subdomain. Fill in the appropriate information for the subdomain (See pic below, click on image for larger version ).
In the subdomains field, enter the name of the subdomain you would like to create. In the example screenshot, I could enter http://subdomain.yourdomain.com , in a browser to see this new subdomain site.
The document root is where your files for the new subdomain will be stored. Each subdomain can be a unique site with its own html files. In the example above, the sudbdomains document root is based off the public_html file structure with the new subdomain appended. You will have to create this folder for the subdomain yourself, cPanel does not create this for you.
If you have subdomains that are already setup, they will be listed at the bottom of the page. Here you can modify the information related to each subdomain that has been added.