A "normal" is a unit (length one) vector pointing perpendicular to a surface.

If the normal vector points away from the camera, the corresponding surface is not visible, and can be ignored.

If the normal vector points away from a light, that light does not illuminate the surface. If the light does illuminate the surface, the light location, normal vector, and camera location are considered together with the properties of the surface (matte, shiny, etc.) to determine the appropriate amount of illumination.

Per face (triangle), per vertex, or via a normal map. If per face, the normal is considered to be constant over the triangle. If per vertex, the normal is interpolated over the triangle. If via a normal map (special texture encoding the normals over a surface), the UV coordinates are used to determine the appropriate normal to use at each point of the face.