There are 2 services you need for a functioning site - a domain plus a hosting plan for it. Any time you type the domain in your browser, you see the content that’s uploaded in the hosting account, but if that domain is not linked to such an account or to an e-mail service, it's parked. To put it differently, the domain is registered and you're its owner, but it does not have any content of its own. As a substitute, it can open either a pre-made “Under Construction / For Sale” page from the registrar company, or it can be forwarded to some other URL of your choice. The benefit of parking a domain name is that you can keep it and be sure that nobody else is going to take it. In the meantime, it will not block a slot for a hosted domain within your account. You could also park domain names if you have a .com, for example, and you register domain addresses with other extensions such as .net, .org or country-code ones to forward them to the main site in order to protect a brand name.