A subdomain is the section of the web address that is before a domain name and you have almost certainly seen a lot of subdomains while exploring the Web. For example, many websites like 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 an independent website and its own records, so you're able to even host it on another server. The practical use is that you could have a supplementary website, like an e-learning portal for college students as well as 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 specific website, not mentioning that it'll be more safe to have the sites separate from each other.