attenuation is the decrease of light intensty as you go far away from the light source. The reason for this is that light is a radiated energy. as you move a way from the light source (point lights or spherical light sources) the area of the sphere on which this energy is distributed increases. the area increase of this sphere is directly proportional to the square of the radius, so the attenuation for spherical and point light sources as the light bulb is directly proportional to the square of the distance from the lihgt source.BUT this is the true theoritical ratio. You may use another ratio (as making attenuation directly proportional to the radius instead of radius squared) as a hack to compensate for light interreflection (unless you are using radiosity in a non real time appliction).Light interreflection always makes the scene brighter than a normal per vertex lighting.
Note that attenuation doesnot affect directional light sources (as a laser beam).Directional light sources are not affected by the distance.