A lightmap is data describing how the lighting varies by position. They commonly take the form of a texture (i.e. an image) pasted across the game world. Dark interior corners are dark because the lightmap is dark in those places.
Static objects in the environments generally have their shadows "baked" into the lightmap, rather than being drawn in real-time. So if you're on the floor of the first chasm area in AotCR, the shadow from the skybridge is a dark area in the lightmap.