Occlusion is a fast and simple method to simulate the effects of global illumination. Ambient occlusion is most often calculated by casting rays in every direction from the surface. Rays which reach the background or “sky” increase the brightness of the surface, whereas a ray which hits any other object contributes no illumination. As a result, points surrounded by a large amount of geometry are rendered dark, where as points with little geometry on the visible hemisphere appear light. The soft appearance achieved by ambient occlusion alone is similar to the way an object appears on an overcast day. So what basically this shader does is try to re-create the effect of ambient occlussion. It is moderately low on render time, and has significant advantanges in the final look of an image. The mib_amb_occlusion node come with a variety of attributes to customize the final rendered AO image.