A subdomain is the section of the web address which is before a domain and you've probably seen a lot of subdomains while exploring world wide web. For example, many sites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit of employing a subdomain is that it can have an independent site 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 students as well as the main school site. If you use subdomains instead of subfolders, it's going to be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more safe to have the websites separate from one another.