[cubefox]

PBR textures don't include just the usual colored images, but also other surface properties that influence how specific surfaces interact with light. Like roughness, glossiness, elevation (surface normal vectors), whether the surface is a metal, whether it has specular highlights, whether it appears fuzzy, whether it is partly translucent (like skin) and so on. The combination of such textures is called a (PBR) "material".

It's called "physically based" because there exist simple physics formulas for these properties, so it luckily isn't necessary to simulate the all microphysical details that cause them in real objects. Similar to how one can describe a gas with a few parameters from thermodynamics without considering the molecular details that explain those phenomena.

In old 3D renders everything looked like plastic because they had only very primitive surface properties.