What is a Lock master key?

A lock that is not mastered will only work with one key. By adding small pins in the lock cylinder it can be made to use two keys. The second key is a master key. By adding different sizes of these small pins to the cylinders of locks with different keys they can all be made to work with this master key. For a full explanation go to http://home.howstuffworks.com/home-improvement/household-safety/security/question710.htm