The Crown chakra is about wisdom and being one with the world. When this chakra is open, you are unprejudiced and quite aware of the world and yourself.
If it is under-active, you're not very aware of spirituality. You're probably quite rigid in your thinking.
If this chakra is over-active, you are probably intellectualizing things too much. You may be addicted to spirituality and are probably ignoring your bodily needs.