A subdomain is the part of the web address that is before a domain and you have almost certainly seen a lot of subdomains while exploring world wide web. As an example, many sites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of using a subdomain is that it can have an independent website 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 site, such as an e-learning portal for pupils aside from the primary school website. If you are using subdomains instead of subfolders, it'll be much easier to perform maintenance or to upgrade a specific site, not mentioning that it'll be more safe to have the websites separate from each other.