A subdomain is the part of the web address that is before a domain name and you've probably seen a lot of subdomains while surfing around the Internet. For example, many sites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. 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 different server. The practical use is that you can have a supplementary site, 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'll be much easier to perform maintenance or to upgrade a certain site, not mentioning that it will be more safe to have the sites separate from one another.