A subdomain is the part of the web address that is before a domain and you've probably seen a lot of subdomains while surfing around world wide web. As an illustration, many sites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The best thing about using a subdomain is that it can have an independent website and its own records, so you're able to even host it on a different server. The practical use is that you could have a supplementary site, such as an e-learning portal for pupils on top of the main school site. If you use subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a certain site, not mentioning that it'll be more secure to have the websites separate from each other.