In this case it means you park a domain you already own to your account to use, it's just what cPanel decided to call that option I guess, effectively parked domains in cPanel are alias domains, as all parked domains as well as the main domain would show the same content.
In a way you are right though, some registrars offers you to "park" the domain with them if you don't currently have a page for it, by pointing them to an ad page of theirs. As an affiliate program kinda, you get some small amount for every signup that comes through your parked domains address.