A subdomain is the section of the web address that's before a domain and you've almost certainly seen a lot of subdomains while exploring world wide web. For example, many sites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main benefit of using a subdomain is that it can have a separate site and its own records, so you can even host it on a different server. The practical use is that you can have a supplementary site, like an e-learning portal for pupils on top of the main school site. If you work with subdomains rather than subfolders, it's going to be much easier to perform maintenance or to upgrade a particular website, not mentioning that it's going to be more secure to have the sites separate from each other.