A subdomain is the part of the web address that's before a domain and you've quite possibly seen a lot of subdomains while exploring the Internet. As an example, many websites such as 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 a separate website and its own records, so you are able to even host it on a separate server. The practical use is that one could have a supplementary site, like an e-learning portal for students as well as the primary school site. If you use subdomains as opposed to subfolders, it will be much easier to perform maintenance or to upgrade a certain website, not mentioning that it will be more safe to have the sites separate from each other.