A subdomain is the section of the web address that is before a domain name and you have almost certainly seen a lot of subdomains while exploring the Internet. As an example, many sites like Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main advantage of using a subdomain is that it can have a separate website and its own records, so you are able to even host it on a different server. The practical use is that one could have a supplementary site, like an e-learning portal for pupils aside from the primary school website. If you work with subdomains as an alternative to subfolders, it'll be much easier to perform maintenance or to upgrade a certain site, not mentioning that it's going to be more secure to have the websites separate from each other.