A subdomain is the part of the web address that's before a domain and you have almost certainly seen a lot of subdomains while browsing the Web. As an example, many websites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main benefit of employing a subdomain is that it can have an independent website and its own records, so you are able to even host it on a different server. The practical use is that one could have a supplementary website, like an e-learning portal for college students in addition to the main school website. If you work with subdomains instead of subfolders, it will 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.