From BlenderWiki
The following setting depends on how you set the Ambient Light.
Material Ambient Effect
Each object’s material setting (diffuse shader) has an Ambient slider that lets you choose how much ambient light that object receives. Generally, about 0.1 to half is good, depending on the color of the ambient light. A setting of 0.0 means that the object does not receive any ambient light; it is only lit by light that actually gets to it. A setting of 1.0 means that it is lit by all of the world’s ambient light. Increasing the Ambient setting has the effect of flattening the shading, since the ambient light washes out the diffuse shading.
You should set this slider depending on the amount of ambient light you think the object will receive. Something deep in the cave will not get any ambient light, whereas something close to the entrance will get more. Note that you can animate this effect, to change it as the object comes out of the shadows and into the light.
Consequently, this slider also controls how much the material is affected by Ambient Occlusion. For instance, if you do not wish for the material to receive any Ambient Occlusion (lightening, darkening or both), move the slider all the way to the left-most (zero) position.
