How it works...

Post process materials are a bit different than the ones we apply to 3D models. Just as we need UVs in our meshes to indicate how textures should wrap around the objects, we need to access a specific bit of information from the scene that is being rendered in order to apply certain effects to it.

Instead of UVs, though, the information that we need from the scene comes in the shape of different scene textures, such as the one we dealt with before named Post Process Input 0. There are many others, and we will use them according to our needs. For instance, we can also access the subsurface color or the Ambient Occlusion pass if we want to. The point is that we have access to many scene buffers and you can use them to create a material that suits your needs. 

Of course, using those nodes is not always mandatory and you can create a post process material using only the nodes that you would use on a traditional material. However, don's pass on the resources available to you through the Scene Texture parameter.