A subdomain is the section of the web address that's before a domain and you've most likely seen a lot of subdomains while exploring world wide web. For instance, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main advantage of using a subdomain is that it can have an independent site and its own records, so you are able to even host it on another server. The practical use is that you can have a supplementary website, like an e-learning portal for college students in addition to the main school site. If you use subdomains as an alternative to subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more safe to have the websites separate from each other.