A subdomain is the part of the web address which is before a domain name and you've probably seen a lot of subdomains while surfing around the Web. For example, many sites like Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of using a subdomain is that it can have an independent website and its own records, so you're able to 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 aside from the primary school website. 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 sites separate from each other.