A subdomain is the section of the web address that's before a domain name and you have almost certainly seen a lot of subdomains while browsing world wide web. As an example, many sites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The advantage of using a subdomain is that it can have a separate site and its own records, so you are able to even host it on a separate server. The practical use is that one could have a supplementary website, such as an e-learning portal for pupils on top of 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 website, not mentioning that it's going to be more secure to have the sites separate from each other.