If you place a block on a brand/companyname (a string of characters), then no one can register a domain name that contains those strings of characters. They also block up to 500 variations (placing a block on 'paypal' would get 'paypa1' blocked as well.

Those domains that are blocked won't be 'parked', someone trying to register the domain that's blocked, it will just say it's not available for registration.