What is the background node for Stack layout groups about?

I noticed this in the documentation on the Stack layout something about a background:

In an object representing layout.padding there is the background property of type SceneNode:

background sceneNode of NULL SceneNode used as the background or null if no background set

What is the background scene node and how can it be used?

LOVE IT!!! :heart_eyes:
Add a rectangle to a group and then in the layers panel right click and select Make background