Master lock is a combination of password or a pin, that is set up by the user even though he/she first bought the device first and try it first. This is an emergency password to unlock the gadget when you forgot its password. it is using four-digit sequence code to unlock the device. Sim pin can also be the master code if you have set it as the master code for the device. Many times, I forgot my lock screen password but at that time I did not know about this thing but today I have set up my master code password in case of emergency.