Normal maps are a technique used in 3D graphics to emulate surface details on a mesh (3D model). Think of a brick house - if you want to render a detailed close-up view of the wall, the naive approach would be to include the seams between individual bricks in the wall mesh itself and render that. But that takes a lot of effort for the modeler and greatly complicates the mesh in terms of how many polygons it consists of. Alternatively, you can use a flat mesh and specify a normal map - an additional 2D image that, when applied on top of the wall texture, encodes which parts of it are protruding/depressed compared to the base level of the mesh. This way you can model a bunch of buildings using simple boxy meshes and then implant details to the walls using a combination of textures and normal maps.

The project above automatically creates normal maps of metallic surfaces using a camera extension. This is useful for 3D modelers, since they no longer need to have special normal maps manually created for each surface type in a scene.