A subdomain is the part of the web address that's before a domain name and you've probably seen a lot of subdomains while browsing the Internet. For example, many websites like Wikipedia have versions in different 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 different server. The practical use is that you could have a supplementary website, like an e-learning portal for pupils on top of the main school website. If you use subdomains as opposed to subfolders, it will be much easier to perform maintenance or to upgrade a certain website, not mentioning that it's going to be more safe to have the websites separate from each other.