A subdomain is the section of the web address that's before a domain name and you've almost certainly seen a lot of subdomains while exploring world wide web. For instance, many websites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The advantage of using a subdomain is that it can have a separate website and its own records, so you can even host it on a separate server. The practical use is that you can have a supplementary site, such as an e-learning portal for pupils on top of the primary school site. If you use subdomains rather than subfolders, it'll be much easier to perform maintenance or to upgrade a particular site, not mentioning that it will be more secure to have the sites separate from one another.