A subdomain is the part of the web address which is before a domain name and you've probably seen a lot of subdomains while surfing around the Web. As an illustration, many websites such as 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 separate server. The practical use is that one could have a supplementary site, such as an e-learning portal for pupils aside from the primary school website. If you use subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a particular site, not mentioning that it's going to be more secure to have the websites separate from one another.