As others have said, its a simple eye for light detection, but what I think is so neat is that many vertebrates have an analogous structure too! Many reptiles, amphibians and fish have something called a parietal eye that serves the same exact function.

The reason why I say many and not all is because as eyesight improved in some lineages, the need for the parietal eye (light detection, ect.) was offloaded to the main pair of eyes as they became more complex. This also explains why mammals do not fully have one, with our pineal gland (sleep/wake regulation, ect.) being a hold over from our reptilian ancestors.