A subdomain is the part of the web address that is before a domain and you've almost certainly seen a lot of subdomains while browsing world wide web. As an example, many sites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. 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 separate server. The practical use is that you can have a supplementary website, like an e-learning portal for students aside from the primary school website. If you work with subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a particular website, not mentioning that it will be more safe to have the websites separate from each other.