A subdomain is the section of the web address that's before a domain and you have most likely seen a lot of subdomains while exploring the Internet. For example, many websites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of using a subdomain is that it can have an independent website and its own records, so you can even host it on a separate server. The practical use is that you can have a supplementary site, like an e-learning portal for pupils on top of the primary school website. If you use subdomains as an alternative to subfolders, it'll be much easier to perform maintenance or to upgrade a certain website, not mentioning that it'll be more safe to have the sites separate from one another.