A subdomain is the part of the web address which is before a domain name and you've almost certainly seen a lot of subdomains while surfing around world wide web. As an illustration, many sites such as Wikipedia have versions a number of languages using subdomains - en.wikipedia.org, de.wikipedia.org etc. The main benefit of using a subdomain is that it can have a separate site and its own records, so you can even host it on a separate server. The practical use is that you can have a supplementary website, such as an e-learning portal for students in addition to the main school website. If you are using subdomains instead of subfolders, it's going to be much easier to perform maintenance or to upgrade a particular site, not mentioning that it'll be more safe to have the sites separate from one another.