A subdomain is the section of the web address that's before a domain and you have most likely seen a lot of subdomains while browsing the Web. For instance, many websites like Wikipedia have versions in different 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 can even host it on a separate server. The practical use is that you could have a supplementary website, like an e-learning portal for students aside from the main school website. If you are using subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a certain website, not mentioning that it will be more secure to have the sites separate from one another.