This really belongs in "physics" or "optics", but there doesn't seem to be those categories. My best shot, given I'm not a physics professor: Lambert's Law says that the intensity of emmitted light from a surface is directly proportional to the cosine of the angle between the line of view and the normal to the surface. A Lambertian surface is a surface that follows this rule exactly. In practice, most surfaces are not perfectly Lambertian. This means that the light percieved from a Lambertian surface is constant, because while the radiant energy is smaller as the viewing point goes off 90° (straight-on; viewing angle = 0°; cosine = 1), the apparent area of the surface from that point of view is larger, so the same amount of light is percieved by the viewer.