Registries require the use of name servers or hosts for every domain registered. Parking is the process by which someone selects a domain name, and "parks" it by registering the domain name under someone's name servers. The registrar normally has default name servers that can be used. Parking can be done by anyone, to anyone else who has active name servers. However, parking a domain name alone will result in no service (web hosting, e-mail) for that particular domain name.
Parking a domain may also refer to adding it as a parked domain in cPanel. This will configure our server and create DNS records on our name servers for it to load the same files as the main domain on the account. A video showing how to do this can be found at http://www.cpanel.net/media/tutorials/parked.htm.