Directional lights have only color and direction, not position. They give off parallel light, meaning that all light generated by it travels through scene in the same direction. You can imagine a directional light as a light source at near infinite distance, such as the sun. Directional lights are not affected by attenuation or range, so the direction and color you specify are the only factors considered when Direct3D calculates vertex colors. Because of the small number of illumination factors, these are the least computationally intensive lights to use.