This redstone device allows you to create password locked doors, secret areas and hidden passages. The “password” in this case would be an item thrown into a hopper. If the right item is thrown into the hopper, then the password is accepted. If the wrong item is entered, then the system throws an error. This is much more sophisticated than the old “pressure plate locks” that were used in many minecraft adventure maps and really does hold the promise of creating some sophisticated quest systems in the future.