A subdomain is the part of the web address that's before a domain name and you have probably seen a lot of subdomains while browsing the Internet. As an example, many sites such as 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 site and its own records, so you can even host it on a different server. The practical use is that one could have a supplementary site, like an e-learning portal for college students on top of the main school site. If you work with subdomains as an alternative to subfolders, it's going to be much easier to perform maintenance or to upgrade a certain website, not mentioning that it'll be more secure to have the sites separate from each other.