Subdomains allow you to easily manage parts of your website and make your link look great at the same time! For example, instead of having the introduction page of your site look something like this - mysite.com/a/intro - you can just create a subdomain for that part of your website - intro.mysite.com. Subdomains can also act as fully independent websites.
To create a subdomain, access your hosting account's hPanel and locate the Subdomains menu in the Domains section.
In the Create a New Subdomain section you can choose which domain will be used as your subdomain or simply create a new subdomain.
Below that you will notice the List of Current Subdomains section. Here you can delete your subdomain or check it's root (main) directory - your subdomain will load content from that directory specifically :)