The moon reflects so much sunlight that it can be hard for your eye to home in on details when you're viewing it through your telescope. The Celestron Moon Filter screws onto the insert barrel of most 1.25" eyepieces and cuts the glare, heightening contrast so you can pick out fascinating features like craters and mountains with ease. It's not just for the moon, either. A bright planet like Venus can be viewed more easily with the Moon Filter in place, as well.
NOTE: The Moon Filter is not sufficient for viewing the sun.