A Bump Map is a method of creating detail but its fake. These maps create an illusion of depth on the surface of a model using simple light tricks, no additional resolution is added to the model as the result of a Bump Map.
Bump Maps are limited to 8 bits of colour this means only 256 variants of black grey or white. These values are used to tell the 3D Software 2 things. Up or down. When the values of the bump map are close to 50% grey there is little to no detail that comes through on the surface as the values get brighter/ closer to white details appear to be pushing towards the surface.
High poly chest, no bake.
Low poly chest, with bake.