Look at your eyes in a mirror somewhere that lets a lot of light fall on your eyes. Notice the size of each pupil, which is the black dot in the center of each eye. Next, cup your hands around your eyes to cast a shadow over them. Can you see the pupils becoming slightly larger? Take your hands away. Can you see the pupils shrinking?
The colored part of the eye is a sheet of muscle called the iris. The pupil is a hole in the iris that lets light into your eye. (Both are protected by a glassy covering.) In bright light, the iris automatically tightens to make the pupil smaller and let in less light. In dim light, the iris relaxes to make the pupil larger and let in more light.