A subdomain is the part of the web address that is before a domain and you've probably seen a lot of subdomains while browsing world wide web. For example, many websites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of employing a subdomain is that it can have a separate website and its own records, so you're able to even host it on another server. The practical use is that one could have a supplementary website, like an e-learning portal for college students on top of the primary school website. If you use subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a certain website, not mentioning that it will be more safe to have the websites separate from one another.