The Cosine Mix node is used for mixing two textures together (according to a cosine wave). In the figure1, a Turbulence Texture is combined with a Gaussian Spectrum using the Cosine Mix Texture connected to the Diffuse component of an Octane material. The Amount slider is used to balance the strength of the two textures.
Figure 1 : A Cosine Mix texture node is used to mix a Turbulence texture and an Octane RGB Spectrum Texture. The result is connected to the diffuse channel of an Octane material.