Bulletproof is a means of keeping files from being deleted, like when you go to a website to download a link and find that the link is dead. When you have a "Bulletproof" file what you have is a file that cannot be deleted by anyone, or any government because it is now a torrent. Free to be posted in all countries with no fear of being deleted because they are hosted in countries like China, Serbia, Russia or other free countries without reservation for the whole world to see and obtain.
Let's take a single file posted in a forum, the owner of said file lodges a complaint against the file host like "RapidShare" then "RapidShare" kills the link. But being "Bulletproof" means that it can't be touched or forced by any law because the location is another country that will not and does not subscribe to our laws where the link would otherwise be illegal and or killed.