Ambient maps work in a similar fashion to Lightmaps with the exception that direct lighting from Sun, Spots lights and Omni lights still affects the scene. Ambient maps will bake the overall lighting and environment illuminations in to the objects.
Unlike lightmaps shadows or direct light wont be casted to the texture.
Explorer tree / Static Objects / Bake Ambient maps
Here is an example of the same scene, image to the left has standard lighting + direct illumination, in this case the sunlight.
Image in the middle has ambient lights baked, no direct lighting is visible.
And last image to the right contains a collaboration between ambient lights and direct illumination.
The sunlight or any other light source can be placed in any direction, and unlike lightmaps shadows will update in real time.