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 browsing the Internet. For instance, many sites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The best thing about using a subdomain is that it can have a separate site and its own records, so you are able to even host it on a separate server. The practical use is that you can have a supplementary site, like an e-learning portal for students aside from the main school website. If you work with 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 websites separate from one another.