A subdomain is the part of the web address that is before a domain and you've most likely seen a lot of subdomains while exploring the Internet. As an illustration, many sites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of using a subdomain is that it can have a separate site and its own records, so you can even host it on another server. The practical use is that one could have a supplementary site, like an e-learning portal for students in addition to the main school site. If you work with subdomains as an alternative to subfolders, it's going to be much easier to perform maintenance or to upgrade a particular site, not mentioning that it's going to be more safe to have the sites separate from each other.