A subdomain is the part of the web address that's before a domain and you've most likely seen a lot of subdomains while surfing around world wide web. For example, many sites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit of employing a subdomain is that it can have an independent site and its own records, so you are able to 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 on top of the primary school site. If you use subdomains as opposed to subfolders, it's going to 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 one another.