Lock Mirror Up for Cleaning
Explanation: The only choice in this menu is „Start“ and is for the purpose of cleaning the image sensor.
After confirming it the mirror will be raised and if you have taken off the lens you can look straight onto the sensitive sensor. The mirror will flip back when the camera is switched off.
Tips: A cleaning of the image sensor is described in the Nikon manual but any wrong touch of the sensor can damage it. Be careful!