A subdomain is the part of the web address that is before a domain name and you've almost certainly seen a lot of subdomains while exploring the Web. For example, many sites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main advantage of using a subdomain is that it can have an independent 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 site, such as an e-learning portal for students aside from the primary school website. If you use subdomains as opposed to subfolders, it will be much easier to perform maintenance or to upgrade a particular site, not mentioning that it's going to be more safe to have the websites separate from one another.