A subdomain is the part of the web address which is before a domain and you have probably seen a lot of subdomains while surfing around the Internet. As an illustration, many sites 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 an independent website and its own records, so you can even host it on another server. The practical use is that you can have a supplementary website, like an e-learning portal for college students aside from the primary school site. If you are using 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.