From "Why Meditate"

Awareness makes it possible for us to perceive phenomena of every kind. Buddhism describes this basic quality of the mind as luminous because it illuminates both the external world through perceptions and the (internal) inner world of sensation, emotion, reasoning, memory, hope, and fear.
Although this cognitive faculty underlies every mental event it is not itself affected by any of these events. A ray of light may shine on a face disfigured by hatred or on a smiling face; it may shine on a jewel or on a garbage heap; but the light itself is neither dirty or clean. Understanding that the essential nature of consciousness is neutral shows us that it is possible to change our mental universe. We can transform the content of our thoughts and experiences. The neutral and luminous background f our consciousness provides us with the space we need to observe mental events rather than be at their mercy. We then also have the space we need to create the conditions necessary to transform these mental events.