A subdomain is the part of the web address that is before a domain and you've quite possibly seen a lot of subdomains while exploring the Internet. For example, many websites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main advantage of using a subdomain is that it can have a separate website and its own records, so you can even host it on a separate server. The practical use is that one could have a supplementary website, such as an e-learning portal for students aside from the primary school website. If you work with subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a certain website, not mentioning that it'll be more secure to have the websites separate from each other.