A master keyed system gives you a master key that can open a number of locks which have their own individual keys. These systems are usually used in conjunction with the Restricted (do not copy) style keys but can be on a normal key as well. For example, an office building tenanted by a company with a number of individually occupied offices, with each of these offices having its own key which only accesses their respective office.  The master key would then have access to all of these offices and generally be held by the head of the company, security and/or the cleaner.