A subdomain is the section of the web address that's before a domain name and you have quite possibly seen a lot of subdomains while surfing around world wide web. For example, many websites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main benefit of employing a subdomain is that it can have a separate site and its own records, so you can even host it on another server. The practical use is that one could have a supplementary site, like an e-learning portal for students on top of the primary school website. If you use subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it's going to be more safe to have the sites separate from each other.