The MaterialThe representation of the surface or volume properties of an object. Passes will separate material characteristics into individually rendered elements (figure 1).
Figure 1: Material passes in the Render PassesRender passes allow a rendered frame to be further broken down beyond the capabilities of Render Layers. Render Passes vary among render engines but typically they allow an image to be separated into its fundamental visual components such as diffuse, ambient, specular, etc.. section of the Render Settings.
Figure 2 shows an example of an Octane material separated into its Beauty pass and its Roughness Material pass.
Figure 2: A roughness material pass.