A subdomain is the part of the web address that's before a domain and you've probably seen a lot of subdomains while browsing the Internet. As an example, many sites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The best thing about employing a subdomain is that it can have a separate website 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 website, such as an e-learning portal for pupils on top of the main school site. If you are using subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a certain website, not mentioning that it's going to be more safe to have the websites separate from each other.