A magnetic keyed lock is a locking mechanism whereby the key utilizes magnets as part of the locking and unlocking mechanism.
A magnetic key would use from one to many small magnets oriented so that the North / South Poles would equate to a combination to push or pull the lock's internal tumblers thus releasing the lock. This is a totally passive system requiring no electricity or electronics to activate or deactivate the mechanism. Using several magnets at differing polarity / orientations and different strengths can allow thousands of different combinations per key. 
Traditional lock picking is impossible due to the tumblers being magnetically operated instead of via a physical up and down action. Magnetic keys also cannot be reproduced by locksmiths by sight or other "human sensed" information.