A subdomain is the part of the web address which is before a domain and you've most likely seen a lot of subdomains while browsing world wide web. For instance, many websites such as Wikipedia have versions in various 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 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 college students on top of the primary school site. If you use subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a certain site, not mentioning that it's going to be more secure to have the websites separate from one another.