A subdomain is the section of the web address that's before a domain name and you've almost certainly seen a lot of subdomains while exploring world wide web. As an example, many websites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of employing a subdomain is that it can have a separate website and its own records, so you are able to even host it on another server. The practical use is that you can have a supplementary website, like an e-learning portal for college students on top of the primary school website. If you are using subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a specific website, not mentioning that it's going to be more safe to have the websites separate from each other.