A virtual 3D model is just a shape, the surface is how it appears. The way something looks is part of telling a story. You can tell Mater is an old car from his rusty brown surface and how his life is different from Lightning McQueen with his shiny red surface.
Surfacing artists construct each aspect of an object’s appearance using computer programs called shaders. They break down a complex appearance into specific elements. One shader may define Mater’s underlying brown color, another his rough texture, and yet another puts the gleam in his eyes.