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The Eyeball
Your eye is a very complicated piece of the human anatomy, and is comprised of a number of systems and processes. The eyeball allows you to see what is around you, but in order to do this, the eyeball requires light. Light travels to your retina through your cornea, pupil and lens. Your pupil dilates, or widens, to let more light into your eye, and closes to allow less light into your eye.
The Retina
Your retina contains photoreceptors. Photoreceptors are made up of rods, which respond to dark conditions, and cones, which respond to bright conditions. The light that travels through your eyeball to your retina activates these photoreceptors, enabling you to see in both light and dark conditions.
Rods and the Dark Movie Theater
Rods outnumber cones in your retina by about 20 to 1; on average, each retina contains 120 million rods and 6 million cones. Rods are used in dark conditions because they amplify any and all sorts of light stimuli. They help you see in a dark movie theater, for example, because they are exaggerating the minimal light already present. Additionally, your pupils will dilate to allow more light to stimulate your retina.
Rods and the Bright Sunshine
Because rods amplify light, making it possible for you to see in the dark to a certain extent, the sudden transition from a dark movie theater into bright sunshine will cause the rods in your retina to amplify the light, since they were engaged seconds ago in the theater. Your rods become "saturated" and are no longer engaged.
Cones and the Bright Sunshine
It takes few minutes for your eyes to adjust to the bright sunshine. When your rods stop working, the bright light engages your cones, which will restore your vision. At this point, your pupils will close to prevent too much light from overstimulating your retina.