In other words, a sub-domain is another name for a directory (or folder) of your website (the domain).
For example, if your domain is called yourdomain.com, then you could create a sub-domain called mysubdomain.
The URL for your domain looks like this:
The URL for your sub-domain will look like this:
The sub-domain URL will present the content of the directory called subdomain in the visitor's browser.