The term 'ambient occlusion' refers to how bright light shines on various surfaces. AO is depending on the surface's surroundings.
The calculation is done by projecting rays from the surface in all directions.
The brightness of the surface is boosted by focussing the background light.
On the visible hemisphere, points with little geometry will seem light.
Ambient occlusion is a shading and rendering technique used in 3D computer graphics, modelling, and animation to calculate how exposed each point in a scene is to ambient lighting. Ambient occlusion can be thought of as a determined accessibility value for each surface point.