What is the Ambient Light Sensor(ALS) in NP900X3C?

ALS (Ambient Light Sensor) is located on the top right corner of the keyboard (shown below). 

This sensor is responsible for detection the ambient illumination of the PC. If the ambient light becomes dark, the LCD automatically becomes dim and the keyboard backlight is lit.

On the contrary if the ambient light becomes bright the LCD becomes brighter and the keyboard backlight is turned off.

Press the FN + F1 key combination and select Easy Settings > Display and check the box to turn the illumination sensor ON/OFF.