Bump mapping uses a black and white map to map extra 3D detail onto the surface of a material. The black and white defines topographic height which the rendering engine turns into a 3D surface. It’s used when you have very fine amounts of detail that would be difficult to make with actual geometry. If you think of a metal surface as a flat plane… then the bump map would define the small nicks and dents in the surface of the metal.
There are other greyscale maps you add to get extra realism from a 3D surface:
Specular maps
Opacity maps
Alot of them are used in game modelling because it reduces the load on the game engine, but you can still get a higher level of realism.