Physically Based Rendering, which is a more realistic way for materials to react to the changing light conditions, which result in much more realistic graphic, more similar to those found in the latest game engines.
It was the main new feature added to P3D 4.4 but, in order to see the difference, add-ons must be updated to get advantage of it, textures must be remade and usually the modelling as well, to get the best results.
Which is exactly what we haven being doing for the past 6 months: update GSX to support PBR.