Directional Light - All objects with the same shader applied are lit with the same direction and brightness. The brightness is a fraction calculated from the dot product of the direction of the light and the normal of a particular surface. If the direction vector of the light is normalised ( a unit vector ) it's magnitude will be one, therefore the dot product will result in the fraction cos(theta) where theta is the difference  in angle of the light and the normal to the face.
The surfaces also are coloured by a very dim ambient light, a base colour and brightness. This helps to define what would otherwise be pitch black surfaces. This is a cheep and easy method for lighting these surfaces that would otherwise only be lit by very expensive techniques such as ray tracing.