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The tapetum is a metallic layer of the eye made up of membranes. It is located behind the retina, and only animals have one, not humans. It is the part of the eye that allows animals to see at night. The tapetum is located on the inner part of the eye thus you can not see it without turning an eye inside-out.

The way that light allows animals to see as night is as follows. Light gets projected into the eye and into the area with the tapetum. Because it is metallic, the tapetum acts as a mirror and the light moves back and forth thus making the light stronger because it is getting projected onto the retina multiple times. Then the light it projected out of the pupil and an animal can see better in the dark. If you have ever shown a flashlight in a cat's eye, you can see that it glows. This is caused by the extra light that is not being used when the light is being reflected inside the eye. The reason humans do not have tapetum is that there is no reason for us to be able to see at night. There are no predators that will try and kill us when its dark and this is the main purpose of the tapetum for animals.