A subdomain is the section of the web address which is before a domain name and you have most likely seen a lot of subdomains while exploring the Internet. As an illustration, many sites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of using a subdomain is that it can have a separate site and its own records, so you can even host it on a different server. The practical use is that you could have a supplementary website, like an e-learning portal for pupils as well as 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 particular site, not mentioning that it'll be more secure to have the websites separate from one another.