Just as with human beings, cats' eyes dilate to allow more light in. Their pupils are capable of dilating much more than human beings' eyes; that's why they can see in areas that have very little light.
The pupil size and shape are controlled by the iris muscles. In cats, (like most nocturnal hunters), the iris contracts into a slit in bright light to limit retinal light exposure. The smaller aperture also improves acuity and depth of field. In dim light, the cat's irises open, allowing the pupils to dilate to almost 90 percent of the irises area to catch all the light it can.