BRDF stands for “Bidirectional Reflectance Distribution Function”. It is a mathematical description of a surface finish, or rather, how light reflects off a surface.
If a surface is pretty evenly rough (or evenly smooth), you can usually get a BRDF measurement of it and create an optical model from that. Every optical simulation software has its own method of importing and formatting these models and usually, you’ll have to have a custom model made for each optical software being used and for each material. If you’re lucky, the model you need might already be in the software’s library. Once you have a working model in your optical software, you can simulate the surface texture in your simulations, which is often super important to get right.