Subdomains allow you to manage parts of your website very easily and make your link look great at the same time! For example, instead of having the store page of your site looke something like this: www.example.com/a/store you can just create a subdomain for that part of your website: store.example.com. Subdomains can also act as fully independent websites.
To create a subdomain, access your websites Control Panel and locate the Subdomains menu.
In the Subdomains menu, take a look at the Create a New Subdomain section. You can choose which domain will be used in the subdomain's creation as well as the name for the subdomain there.
You will also notice the List of Current Subdomains section on the same page. Your created subdomains can be deleted here. It's also very useful for checking your subdomain's root directory (main directory). In other words, your subdomain will load content from that directory specifically.