A subdomain is the part of the web address which is before a domain and you've probably seen a lot of subdomains while surfing around the Internet. As an illustration, many websites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main benefit of employing a subdomain is that it can have an independent website and its own records, so you're able to even host it on a different 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 site. If you use subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a particular website, not mentioning that it'll be more safe to have the sites separate from each other.