The Ambient Cubemap lights the scene from a provided image. The image is mapped to a sphere in the far distance (implemented as a cubemap texture with the mip maps storing preblurred versions of the image).
The blurry versions are computed in a way that they can be used for specular lighting with varying glossiness (sharp vs. blurry reflections) and they can be used for diffuse lighting as well.
This effect is independent of the position a material is lit from. Viewer position, material roughness (for specular effects), and the material surface normal are all taken into account.
The Ambient Cubemap has very limited local shadowing (SSAO only). Use a Sky Light instead when representing the sky's lighting.