Node Description and Purpose: The Merge Shader allows you to merge two separate shaders (or even entire shader networks) based on the input of a 3rd shader acting as a mask. You can control the amount of contribution of each input shader as well the method used for merging.
A shader is a program or set of instructions used in 3D computer graphics to determine the final surface properties of an object or image. This can include arbitrarily complex descriptions of light absorption and diffusion, texture mapping, reflection and refraction, shadowing, surface displacement and post-processing effects. In Terragen 2 shaders are used to construct and modify almost every element of a scene.