The pupil is the part of your eye that controls how much light gets in. When we enter in an area of bright light, size of pupil gets smaller (contract) because to control the amount of light.

Additional information:

While entering the dark room pupil becomes large (expands) so that more and more light can enter the eye. Size of the pupil is controlled by iris. Thus we can say that iris controls the amount of light entering into the eye.