A subdomain is a subsection of your website that can exist as a new website without a new domain name. Use subdomains to create memorable URLs for different content areas of your site. For example, you can create a subdomain for your blog that is accessible through blog.example.com and www.example.com/blog.
To set one up, go to the DOMAINS section of your cPanel and click the subdomains icon.
In the below example, we're setting up a subdomain on the domain testdomain.io - Once submitted, the subdomain support.testdomain.io will be functional and will load the contents of testdomain.io/support.
After the subdomain is created, you have the ability to change what folder the subdomain pulls the content from. You also have the ability to override that and have the subdomain function as a redirect to another URL using the manage redirect function.