First off, don't be intimidated. This looks a lot more complicated than it actually is.
Just like many stacks, Sections can have a background applied behind the content that you drop in. The is the Background Layer.
The Background Overlay is primarily used to overlay a transparency color or gradient over background images but can be used for anything that the main background supports.
In addition, Sections Pro has a Layer child stack, (accessed via the blue plus button in edit mode). This Layer allows special content to be used as the Section background, instead of or in addition to, the other layers.
The Layer child can contain a background video or a stacks drop zone for any content that you want. Obviously, things like Impact slideshows are best as backgrounds.
The Layer child also has its own background which you can assign to a color, gradient or image types. This can also act as a poster image for videos.
REMEMBER You don't have to use any or all of these layers. Anything that you do not use will write ZERO CODE to your page and so there is no penalty for having this facility.