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 world wide web. For example, many websites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The best thing about employing a subdomain is that it can have an independent website and its own records, so you can even host it on a separate server. The practical use is that you can have a supplementary site, like an e-learning portal for pupils on top of the primary school website. If you are using subdomains instead of subfolders, it's going to 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 one another.