A subdomain is the part of the web address which is before a domain and you have quite possibly seen a lot of subdomains while exploring the Internet. As an example, many sites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main benefit of using a subdomain is that it can have an independent website and its own records, so you can even host it on a different server. The practical use is that you could have a supplementary website, like an e-learning portal for college students aside from the primary school website. If you are using subdomains rather than subfolders, it's going to be much easier to perform maintenance or to upgrade a certain site, not mentioning that it'll be more secure to have the websites separate from one another.