Domain parking (at least in the context as it's used here) refers to the process of adding additional domain names to a hosting account which access the same content.
A common example of domain parking would be a company who purchases the rights to theircompanyname.com and theircompanyname.net and wishes to have users be able to get to their site via either domain.
For futher information:
Please refer to the cPanel documentation for Parked Domains, here.
