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 with all of the domain names resolving to the account's index page.
Example:
processor.com -> http://processor.com/index.html
cpu.com -> http://processor.com/index.html
All our accounts include the ability to park extra domains, again with all of the domains directing traffic to a single index page (i.e. one website which can be accessed using multiple domain names).
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.