A hidden lock is one fitted on the inside face of the door without leaving anything on show outside: no cylinder, no escutcheon and no keyhole. From the street the door looks as if it only has its usual lock.
It works as a second lock. Anyone who manages to open the main lock finds the door still will not move and has no idea where the other locking point is. It is useful for when you are at home, for patio doors or for doors whose appearance you do not want to change.
There are mechanical models and others worked with a remote or a mobile phone; the latter are electronic locks and we do not work with them. You will find the mechanical ones under hidden locks.