A subdomain is the part of the web address that's before a domain name and you've most likely seen a lot of subdomains while browsing the Internet. For instance, many websites such as Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The advantage of using a subdomain is that it can have an independent site and its own records, so you're able to even host it on a separate server. The practical use is that one could have a supplementary website, like an e-learning portal for pupils in addition to the main school website. If you are using subdomains instead of subfolders, it's going to be much easier to perform maintenance or to upgrade a specific site, not mentioning that it will be more safe to have the websites separate from each other.