The Side node assign black or white values based on the normal direction of a polygon object. This is useful for assigning different textures or materials to different sides of a polygon object. Side node only one option that inverts the normal direction.
HOW TO USE
Let's say you have a polygon object and you want to color it on both sides separately. That's where Side node works. Prepare the setup as you see in the picture below. In this setup, two separate RGB Spectrum nodes were used for each side of the polygon object. "Mix texture" was created to use these 2 colors together with the Side node (not to mix). You can swap colors if you wish, thanks to the invert option on the side node. It is a simple setup and you can use it for many scenarios.