A subdomain is the section of the web address that's before a domain and you have almost certainly seen a lot of subdomains while browsing the Internet. For instance, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The best thing about using a subdomain is that it can have a separate website and its own records, so you are able to even host it on another server. The practical use is that you could have a supplementary site, like an e-learning portal for college students as well as the main school site. If you are using subdomains as opposed to subfolders, it will be much easier to perform maintenance or to upgrade a specific website, not mentioning that it's going to be more safe to have the websites separate from each other.