A subdomain is the section of the web address which is before a domain and you have most likely seen a lot of subdomains while surfing around world wide web. For example, many sites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The advantage of employing a subdomain is that it can have an independent site and its own records, so you can even host it on a separate server. The practical use is that you could have a supplementary website, such as an e-learning portal for students in addition to the main school site. If you use subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a specific website, not mentioning that it will be more secure to have the sites separate from each other.