A subdomain is the part of the web address which is before a domain and you have almost certainly seen a lot of subdomains while browsing 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 advantage of using a subdomain is that it can have a separate site and its own records, so you are able to even host it on a different server. The practical use is that you can have a supplementary website, such as an e-learning portal for students as well as 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 specific website, not mentioning that it'll be more safe to have the sites separate from each other.