Mirror lockup is the technique that can be used to try and reduce the share introduced by mirror slap. In other words, by enabling mirror lockup, we can try and reduce the share introduced by the camera when it moves the mirror away from the sensor, which then takes the picture. Usually, the action of moving the mirror out of the path of light that falls on the sensor is called mirror slap and introduces a small movement in the camera.
When mirror lockup is enabled, the first time we press the shutter release button, the mirror is locked up, out of the way of the sensor. To take the picture, we need to press the shutter release button once again.
To enable mirror lockup, the steps are Settings 4 > Custom Functions > 5: mirror lock up > 1: Enable
To disable mirror lockup, the steps are Settings 4 > Custom Functions > 5: mirror lock up > 0: Disable