The Mix texture mixes two textures together (figure 1). By default, a float value controls the Amount. A value of 0 means the First Texture is visible, and a value of 1 means the Second Texture is visible. Values in-between blend the two textures together in a linear fashion. The Mix texture is similar to the Cosine Mix texture, except for the behavior of the mix slider. In the following illustration, the NodeGraph Editor window is used to clearly illustrate the association of the various texture nodes.
Figure 1: A Mix texture blending Marble and Checks textures together