A subdomain is the section of the web address which is before a domain and you have almost certainly seen a lot of subdomains while surfing around the Web. As an example, many websites such as Wikipedia have versions in different 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 website and its own records, so you are able to even host it on a separate server. The practical use is that you could have a supplementary site, such as an e-learning portal for college students as well as the primary school website. If you use subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a certain website, not mentioning that it will be more safe to have the sites separate from each other.