Before starting our journey into texture mapping, it is important to understand how diffuse materials work. Certain objects might have a uniform color and smooth surface, but are not smooth enough to shine in reflected light. These matte materials are best represented with a Diffuse Shader. While, in the real world pure diffuse materials do not exist, Diffuse Shaders are relatively cheap to implement and are largely applied in games with low-poly aesthetics, so they're worth learning about.
There are several ways in which you can create your own Diffuse Shader. A quick way is to start with Unity's Standard Surface Shader and edit it to remove any additional texture information.