A subdomain is the section of the web address that's before a domain and you have almost certainly seen a lot of subdomains while browsing the Web. For example, many websites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of using a subdomain is that it can have an independent website and its own records, so you're able to even host it on a separate server. The practical use is that you can have a supplementary site, like an e-learning portal for students aside from the primary school site. If you use subdomains as an alternative to subfolders, it'll be much easier to perform maintenance or to upgrade a specific website, not mentioning that it will be more secure to have the websites separate from each other.