There is another lesson very simliar to this lesson. I would recommend checking out This lesson from Blender Intermediate Part 3.
If a material is using a normal map. That will change the way how light is affected on it. If the texture is bumpy, then the lighting should also look bumpy, and not a flat piece of paper.
Specular Maps will show reflections on the material based on the bumpyness of the texture.
Ambient Occlusion Maps
Ambient Occlusion is areas on the map where the light does not reach and causes a smooth shadow. From darkest shadows in the darkest corners and light shadows as it's closer to the light.
A Maps Generator
You can click and drag your texture files into here and then download all the maps that you need.
You can adjust the strength, blue, level, and sharpness level.
Once you are done, click All to download all.
This will download all types of texture maps you need. Then put them all into your Unity project.
NOTE: If the textures appears to be completely black, Re-do it from the beginning when clicking and dragging your texture in.
Ambient Occlusion goes into the Occlusion slot
NormalMap goes into Normal Map slot
SpecularMap goes into the Metallic slot
And it's optional for the Displacement map to go into the Height map slot if you want.
Note: When you click and drag a normal map for the first time, it'll ask you to "Fix Now", Go ahead and click that.