A common expression in astronomy, which means 'looking without any instruments.'

When you go out and just look at the sky, you are doing 'naked eye observing.'

It is also a measure of how hard (or easy) it is to see an astronomical object. For example, the only deep sky object you can see with your naked eye (and the farthest away you can look, about 2 million light years) is M31, also known as The Andromeda Galaxy.