A subdomain is the part of the web address that's before a domain and you have probably seen a lot of subdomains while browsing the Internet. As an illustration, many websites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The 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 another server. The practical use is that you can have a supplementary website, such as an e-learning portal for students on top of the primary school site. If you work with subdomains rather than subfolders, it's going to be much easier to perform maintenance or to upgrade a certain website, not mentioning that it's going to be more secure to have the websites separate from one another.