A subdomain is the section of the web address which is before a domain name and you have quite possibly seen a lot of subdomains while exploring the Internet. For instance, many websites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main benefit of using a subdomain is that it can have a separate website and its own records, so you can even host it on a different server. The practical use is that you can have a supplementary site, like an e-learning portal for pupils on top of the main school website. If you are using subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a certain website, not mentioning that it will be more secure to have the websites separate from each other.