A subdomain is the part of the web address that's before a domain name and you have almost certainly seen a lot of subdomains while exploring the Web. As an illustration, many sites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of using a subdomain is that it can have a separate site and its own records, so you are able to even host it on a separate 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 as an alternative to subfolders, it will be much easier to perform maintenance or to upgrade a certain site, not mentioning that it's going to be more secure to have the websites separate from one another.