A normal map doesn’t contain lighting information, it essentially contains an ‘offset’ to the normal of a surface for each pixel.
Basically, 0.5,0.5,0 (or 128,128,0 - corresponding to X,Y,Z) is ‘flat’ meaning the normal map will not affect how light hits the surface. If you start adjusting the first two values (X, Y) then the light hitting it will think the surface is angled in a different direction. Hence, you can use it to add detail like bumps.