A subdomain is the section of the web address which is before a domain name and you have probably seen a lot of subdomains while exploring the Web. For instance, many websites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main benefit of using a subdomain is that it can have a separate website and its own records, so you're able to even host it on a separate server. The practical use is that you can have a supplementary website, such as an e-learning portal for students on top of the main school site. If you use subdomains as opposed to subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it'll be more secure to have the sites separate from one another.