This is a light sensor that has an adjustable threshold set via a potentiometer. You'd use this on an Arduino digital port to detect when light exceeds a certain value. For example, you could use it to make a project that only operates in the dark (i.e. a night light) or only triggers when the lights come on. You can use the potentiometer to set the exact threshold. Line following is another application, you would use the potentiometer to calibrate line detection.
NOTE: This is not the sensor shipped with Vorpal the Hexapod, which is an analog light sensor that returns a range of values.