A subdomain is the section of the web address which is before a domain and you've most likely seen a lot of subdomains while exploring the Internet. As an example, many websites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of using a subdomain is that it can have a separate website and its own records, so you are able to even host it on another server. The practical use is that you could have a supplementary site, such as an e-learning portal for college students on top of 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 website, not mentioning that it'll be more secure to have the websites separate from each other.