bilinearly interpolated surface normal

I am working on a recreation of a wind simulation. A variable I need is a “bilinearly interpolated surface normal.” The two questions I have are how do I interpolate something and how do I calculate a surface normal from one of the terrain generators (any of them really, haven’t chosen one to use.). Everything I’ve read thus far has just confused me. Any help would be great and if this is suppose to go in a different forum let me know.