What is the environmentBrdfSampler of the PBR material about?

It seems that it is a hard coded base64 image.

BRDF stands for bidirectional reflectance distribution function (http://www.cs.utah.edu/~shirley/papers/jgtbrdf.pdf)

The texture contains an “extract” of that function e.g instead of traying to simulate the function which could be complex on real time inside the shader, we simply store analytical values inside a texture and we do a texture look up to get the result of that function

I know well what brdf means. But why do we use this hard-coded texture? Is it special?

It is a lookup table that allows use to not compute the function in real-time