A subdomain is the part of the web address that is before a domain name and you've probably seen a lot of subdomains while browsing the Internet. As an example, many sites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main benefit of 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 separate server. The practical use is that one could have a supplementary website, such as an e-learning portal for college students as well as the main school site. If you use subdomains rather than 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 one another.