A subdomain is one way of organizing and separating content on your site. You’re already familiar with the concept of subdomains, even if you don’t know it. Consider OU’s public Web site at http://ou.edu.
As you browse parts of that site, you’ll notice that the domain changes. When you’re looking at other pages for OU resources, say the site for the University of Oklahoma Libraries at https://libraries.ou.edu, the URL is no longer just ou.edu. Now the root of the url is libraries.ou.edu.
As you can see the domains serve two purposes: they help to organize the site from a technical perspective, but they also serve as indications to the users that they are in a new/different space. As you work on your site, you’re welcome to create as many subdomains as you like, and in each subdomain you can actually create a distinct, individual Web site.