It uses a camera and some software to simulate 3 different kinds of colourblindness.
Red green - by averaging the red and green channels to give a yellow channel (there are two subtley different forms from loosing the red or the green cone in the eye, but the actual sensitivity of the red and green cones are very similar)
Blue yellow - by loosing the blue cones
Black and white - by loosing all colour sensitivity.
You can then look at various things - if you are red-green colourblind you can't see if fruit is ripe, and it is hard to see a tiger behind green grass (tigers and their prey are red-green colourblind).