Some animals such as foxes, badgers and owls have a reflective layer at the back of their eyes, called a tapetum lucidum. This layer increases the amount of light absorbed by the photoreceptors in their eyes, helping them to see better to hunt for food or avoid predators. It’s particularly common in animals who are out after dark, for that very reason!
You can see this layer shining in an animal’s eye, as the light from your torch or car headlights is simply reflected back to you. So in the darkness, all you can see is the bright orbs of their eyes.
We’ve even used this idea to help people stay safe on roads. We have raised ‘cat’s eyes’ along the central line of roads which use synthetic retroreflectors to reflect light from headlights back to the driver, to help guide them.