A master key systems are a group of locks that have a relationship to each other.
The keys are related in a hierarchy with one key at the ‘top’, (the master) which works on all locks, and then as many keys as required at the ‘bottom’ which may only fit into one lock.
The fewer key levels, the more secure the system. This “key system” allows for the control and assess of areas within a building.
In a business environment the a key system allows the limiting or controlling the areas of access by employees.
In your home a master key system allows for one key to open all your doors and locks instead of having multiple keys.