PBR stands for :Physically Based Rendering.
The picture above shows you a material with PBR enabled and disabled. The difference about PBR is that it allows you to crate physical based rendering on your mesh. The shader allows you to create realistic textures onto your mesh to give it a better look then just normal Flat texture.
You will see PBR used in most modern games but for some early access games they stick to basic textures that they took with there Phones or cameras from home. A great example for this is 7 Days To Die.