normal/bump is technique used for faking the lighting of bumps and dents, making a flat texture come alive and look more 3d.

There are a couple of ways to do this:

1. download the free nvdia plugin for photoshop
https://developer.nvidia.com/nvidia-texture-tools-adobe-photoshop

2. Use photoshops own 3d filter normal map. Only included in CC.

I have had best results with photoshops own filter.

Basically I make 2 bump maps a normal and inverted, and mix and delete them to get the stuff I want to pull inwards and outwards, lastly I combine it with the normal map in the asset folder and blend the layers for a desired result.