A subdomain is the section of the web address that is before a domain and you have most likely seen a lot of subdomains while browsing the Web. For example, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit of employing 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 could have a supplementary site, like an e-learning portal for pupils aside from the primary school website. If you use subdomains rather than subfolders, it will be much easier to perform maintenance or to upgrade a certain site, not mentioning that it's going to be more safe to have the sites separate from each other.