A subdomain is the section of the web address that is before a domain name and you've quite possibly seen a lot of subdomains while exploring world wide web. For instance, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. 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 a different server. The practical use is that you can have a supplementary website, like an e-learning portal for pupils on top of the primary school site. 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's going to be more secure to have the sites separate from each other.