A subdomain is part of a web address which is under the main domain, for example name.example.com. Technically, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is just "example.com". Every subdomain can have its own website and records and can even be hosted with a different company if you need to use a certain feature which is not offered by your current service provider. One example for using a subdomain is if you have a company website and an online store under a subdomain where clients can acquire your products. In addition, you can have a forum in which they can comment on the products and by employing subdomains rather than subfolders you will avoid any chance of all websites going down if you perform maintenance, or update one of the site scripts. Keeping your websites separated is more secure in case of a script security breach.
