A subdomain is a part of a web address that's under the main domain, for example name.example.com. Actually, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is only "example.com". Every subdomain can have its own website and records and can even be hosted with a different company if you wish to use a specific feature that isn't offered by your current provider. One example for using a subdomain is if you have a company site and an online store under a subdomain where clients can acquire your products. Additionally, you can have a forum where they can comment on the products and by using subdomains as an alternative to 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 also more secure in the case of a script security breach.
