A subdomain is the section of the web address that's before a domain name and you have most likely seen a lot of subdomains while exploring the Web. For example, many sites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The best thing about using a subdomain is that it can have a separate website and its own records, so you can even host it on another server. The practical use is that you can have a supplementary site, like an e-learning portal for pupils as well as the primary school site. If you work with subdomains as opposed to subfolders, it will be much easier to perform maintenance or to upgrade a certain site, not mentioning that it'll be more secure to have the websites separate from each other.