PBR stands for Physically Based Render, it's a technique for 3D surface shading that tries to replicate the physical properties of light when hitting a given surface. More info: https://en.wikipedia.org/wiki/Physically_based_rendering
One of the main advantages of PBR - besides looking extremely good for minimal effort by the artists - is how streamlined the process of generating the textures becomes. Regardless of if you created the textures \in an image editing program or in a dedicated texturing app like Substance Painter, you know that "what you see is what you get" - ie. the final look of the model in the target gaming engine will be the same as what you're seeing in your authoring software.
SanguinarioJoe (from Spring 1944) & Ivand (from ZeroK) have developed powerful PBR shaders for the Spring engine. Follows the related links: