!til &learningjam2024 The concept of BRDF's turned out simple enough. It's a function that returns the amount of light being reflected based on a given direction pointing towards the viewer and a direction pointing towards the light source. Whether they are vectors or angles based on surface normal seems to vary between sources.
A BRDF is meant to be a self contained thing in the rendering algorithm that can be swapped at will. Basically it's an abstraction.
In practice a BRDF implements some material model. Apparently it's common to combine one for diffuse and one for specular. Any old material model you might have heard of (Lambert, Phong etc) counts as a BRDF. Nowadays BRDF's are usually based on the micro facet model, both in games and film.
This is currently my primary source, which I'm still reading through.
https://boksajak.github.io/files/CrashCourseBRDF.pdf