A subdomain is the part of the web address that's before a domain name and you've probably seen a lot of subdomains while exploring world wide web. As an example, many websites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The advantage of using a subdomain is that it can have an independent website and its own records, so you can even host it on a separate server. The practical use is that you can have a supplementary website, such as an e-learning portal for students in addition to the primary school website. If you use subdomains as opposed to subfolders, it's going to be much easier to perform maintenance or to upgrade a specific website, not mentioning that it will be more secure to have the sites separate from one another.