PBR - Physically Based Rendering
PBR or Physically Based Rendering is all about creating and rendering shaders and models in a realistic, physically based manner. The goal is to create shaders that work in any lighting condition and environment and always behave physically plausible and predictable. This is in contrast to legacy non-pbr shaders that require baked-into-diffuse-map-lighting and speculars-only which did not behave consistently in changing lighting situations.
PBR is the practical execution of previously discussed topics like albedo, fresnel, and linear workflow.
Since this topic is very broad and has been discussed and explained in several very good and thorough guides, I will refrain from trying to do the exact same thing here again. Instead, I will continue to go over related topics and explain those in more detail while listing the most important bullet points here.
It is strongly recommended to read the guides linked below to learn more about PBR in detail:
Things you should know about PBR
After reading the guides referred to above (or the topics on this website), you should be able to explain the following concepts: