A subdomain is the part of the web address that's before a domain and you have probably seen a lot of subdomains while surfing around world wide web. For instance, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The best thing about using a subdomain is that it can have an independent site 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 site, like an e-learning portal for students on top of the main school website. If you use subdomains instead of subfolders, it's going to 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.