The normal map tells us, for each pixel, the “normal” at that point on the surface. The normal is a vector which points perpendicular to the surface. This vector has XYZ components which are encoded as RGB values. The X component is encoded by the amount of red, and so on. The red is in the range `[0, 1]`, and we convert this to the X component in the range `[-1, 1]`.
The surface of the tile is completely matte. It’s like wood, not like metal. This is because the shader uses “Lambertian shading”, which is a model of matte objects. In Lambertian shading, the intensity is proportional to `dot(normal_direction, light_direction)`. We can interpret this formula as answering the question, “is the surface facing the light?”. Note in particular that this formula does not depend on `camera_direction`. The illumination of shiny objects depends on `camera_direction` because light is reflected towards the camera. The illumination of matte objects does not depend on `camera_direction` because the matte surface scatters light equally in all directions.