The current light level as a (massaged) percentage of the minumum and maximum light levels ever seen.
After setting this state up, you should calibrate it by covering up the device for a few seconds and exposing it to bright light.
Warning: on at least some devices, 0% is the level of a dimly lit room, rather than that of total darkness.
See Also: Menu / Prefs / Monitor / Display Off Monitoring / Light Sensor