A subdomain is the part of the web address that is before a domain and you've probably seen a lot of subdomains while browsing the Internet. As an example, many websites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The advantage of employing a subdomain is that it can have a separate site and its own records, so you're able to even host it on another server. The practical use is that you can have a supplementary site, such as an e-learning portal for students as well as the primary school website. If you use subdomains instead of subfolders, it's going to be much easier to perform maintenance or to upgrade a certain site, not mentioning that it'll be more safe to have the websites separate from each other.