Polycode's material system can be used to implement multi-pass post-processing screen shaders and assign them to scene cameras.
Post processing can be used to create screen-based effects, like glare, color correction and depth of field.
On a technical level, it involves rendering the scene to a buffer and then passing it as a texture to a shader applied to a full screen quad. Polycode does all of that automatically when you apply a screen material to a Camera.
A screen material is similar to a regular material, but it can also define multiple passes using different shaders and can define texture buffers to render to.