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 surfing around world wide web. For example, 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 site and its own records, so you're able to even host it on another server. The practical use is that you could have a supplementary website, such as an e-learning portal for students on top of the primary school site. If you are using subdomains as opposed to subfolders, it's going to be much easier to perform maintenance or to upgrade a certain website, not mentioning that it will be more secure to have the websites separate from each other.