The Mix texture mixes two textures together (figure 1). By default, a float value controls the Amount parameter. A value of 0 means the First Texture is visible, and a value of 1 means the Second Texture is visible. Values in-between these numbers blend the two textures together in a linear fashion. The Mix texture is similar to the Cosine Mix texture, except for the mix slider behavior.
Figure 1: The Mix texture node blends a RGB Image and Cell Noise texture together