What do many animals have on the back of their eyeballs to help them see at night?

The tapetum helps them see at night. It is behind the retina. When light goes through the retina, it strikes the tapetum and passes back through the retina, illuminating the image on the retina a second time. This is why it looks like their eyes glow when light hits them at night. The light is escaping out through the pupil.