A hololock was a small device hidden within a structure which could be activated by triggering a sensor, once it was located. At this point, the sensor would activate and begin scanning the bone structure of the user to determine if the individual should have access. Once this was done, five holographic keys would appear which would only be present for a set amount of time to allow the user to input the correct access code, if not done so within the time the keypad would vanish.
In the mirror universe, the mirror Kirk used a hololock to access a hidden corridor on the asteroid prison camp near the Goldin Discontinuity. (ST novel: Dark Victory)