A subdomain is the section of the web address that is before a domain and you have almost certainly seen a lot of subdomains while surfing around the Internet. As an example, many sites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main benefit of using a subdomain is that it can have an independent website and its own records, so you're able to even host it on a separate server. The practical use is that one could have a supplementary site, such as an e-learning portal for students aside from the main school site. If you use subdomains rather than subfolders, it's going to be much easier to perform maintenance or to upgrade a particular site, not mentioning that it's going to be more safe to have the sites separate from each other.