A subdomain is the part of the web address which is before a domain name and you've probably seen a lot of subdomains while browsing world wide web. For instance, many sites such as Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The main advantage of employing a subdomain is that it can have a separate site and its own records, so you're able to even host it on a different 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 work with subdomains as an alternative 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 secure to have the sites separate from each other.