A subdomain is the section of the web address which is before a domain and you have most likely seen a lot of subdomains while exploring the Internet. For example, many sites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of using 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 you could have a supplementary website, like an e-learning portal for pupils aside from the main school site. If you use subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a certain site, not mentioning that it will be more safe to have the sites separate from one another.