A subdomain is the part of the web address that is before a domain name and you've most likely seen a lot of subdomains while browsing world wide web. For instance, many sites like Wikipedia have versions in various 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 are able to even host it on a separate server. The practical use is that one could have a supplementary website, like an e-learning portal for students aside from the main school website. If you use subdomains as an alternative to 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.