Understanding material .mtl files
@PaulHK explained it very well in the comment section:


  Ambient is a (rough) approximation of indirect lighting. For example, an outdoor object would be lit by the sun but the surfaces facing away from sun would be black if using only lambert shading, we can use dark gray for the ambient colour to mimic the effect of indirect lighting from the ground and sky


If you were to visualize this with a shading model, say Phong, the Ka, Kd and Ks values would correspond to the following lighting calculations.