Baader Moon&SkyGlow filter's neodymium glass material filters out the wavelength range of streetlights, making the sky background darker. It also reduces the glare of moonlight and enhances the colours of the planetary sky without altering the tone of the image. By itself, it is poorly suited to correcting colour aberrations in acromatic lenses, and Baader Fringe Killer is recommended as a companion.
In addition to the above, it "cuts off" the UV/IR wavelength range which is particularly beneficial for photographic use.