A light map or lighting map is a bitmap that stores the lighting levels (intensity and color) falling on an object in the scene. Typically, you create a light map by rendering to a texture (texture baking). Light maps are primarily for use in game engines, but you can also use them to speed up renderings.

Banana object in a lighted room

Light map of the banana

If you use the DirectX Manager, you can display light maps interactively in viewports, using either the LightMap shader or the Metal Bump shader.