A scene can be standalone or layered. A standalone scene will be shown exactly as it is while layered scenes will combine one or more scenes to form the final image
Example of a standalone scene
In this scene you can see that it will work just fine as it is on its own.
For a layered scene however we combine up to 3 different layers. For example if we take this background
and this foreground
and a post processing effect that simulates an old scratched movie we get the following
but if we just change the post processing to one that converts the image to gray scale and then inverts it we get this instead
We can change the background scene or the foreground scene the same way to form 1000 of new scenes.