In the real world, the farther an object is away from a light, the less effect that light has on the object. For example, if you look at a street lamp at night (especialy in the fog) you'll be able to see the intensity of the light dropping off away from the lamp. This phenomenon is known as attenuation.
This effect is modelled by OpenGL using an attenuation factor, which can reduce the effect of a lights contribution to the color of an oject based on the distance to the object. The attenuation factor is calulated as follows:
Each factor has a default of (1, 0, 0) which results in no attenuation by default. You can change the factors by passing LightParamater.ConstantAttenuation, LightParamater.LinearAttenuation and LightParamater.QuadraticAttenuation to the GL.Light function. Each of these attenuation factors takes a single floating point number as an argument.
This sample code sets all attenuation factors
GL.Light(LightName.Light0, LightParameter.ConstantAttenuation, 4.0f);
GL.Light(LightName.Light0, LightParameter.LinearAttenuation, 1.0f);
GL.Light(LightName.Light0, LightParameter.QuadraticAttenuation, 0.25f);
Attenuation factor affects only positional light sources, it makes no sense in terms of a directional light as the direcitonal light is infinitaley far away.
OpenGL chose this attenuation model because it is pretty close to how nature actually looks / works. The problem is they lost all intuition with this formula. When setting a point light i expect to be able to say "I want a light with a radious of 5, attenuation should start 2 units away from the center".
There is no intuitive way to set the attenuation of a point light. You have to play around with it until it looks the way you want it to. It's actually so bad, i built an application that renders a sphere on a plane with an attenuated spot light and has sliders for all factors. I play with the sliders until it looks good and copy the values into my code