For an example look at our URL:
https://support.hardhathosting.com

The "support." in front of "hardhathosting.com" is a sub domain. It works the same as a regular domain name.

Sub domain names can be anything you want. Some examples:
"mail.", "sales.", "marketing.", "resume.", "mykids.", "products.", "anything."

Just like with our domain alias option and extra websites, you may also have your sub domain pointed to a different directory.