A subdomain is the section of the web address that is before a domain name and you have quite possibly seen a lot of subdomains while browsing the Internet. For example, many sites like Wikipedia have versions in different languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The main benefit of using a subdomain is that it can have a separate site and its own records, so you're able to even host it on a separate server. The practical use is that you can have a supplementary website, like an e-learning portal for students in addition to the main school website. If you are using subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a certain site, not mentioning that it'll be more safe to have the sites separate from one another.