A subdomain is the part of the web address that's before a domain name and you've probably seen a lot of subdomains while browsing world wide web. As an illustration, many websites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The best thing about using a subdomain is that it can have a separate site and its own records, so you can even host it on a separate server. The practical use is that you could have a supplementary site, like an e-learning portal for college students in addition to the primary school website. If you use subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a certain website, not mentioning that it'll be more safe to have the websites separate from each other.