A subdomain is the part of the web address that's before a domain and you have almost certainly seen a lot of subdomains while browsing the Internet. For instance, many websites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of using a subdomain is that it can have an independent site 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 aside from the primary school website. If you work with subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a particular site, not mentioning that it will be more secure to have the sites separate from each other.