Registering more than one domain name related to the same website means to register the same Domain Name but with a different extension. For example, you can register domain.com, but you can also register domain.org, domain.net, or even some country-specific domain like domain.de, doman.us, and so on. The main goal of registering as much as alternative Domain Names as possible is to avoid someone else doing that with your original Domain. For example, if a customer register mybusiness.com, another customer can register mybusiness.net and have a different website under that domain or even a similar website pretending to be the original business.