A subdomain is the part of the web address which is before a domain and you've probably seen a lot of subdomains while exploring the Internet. As an illustration, many sites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main advantage 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 one could have a supplementary site, like an e-learning portal for students as well as the primary school site. If you are using subdomains as opposed to subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more safe to have the sites separate from one another.