A “background” in Slider Revolution is a type of layer [?]
Layers are containers for visual, audio or layout items and are added to slides. For example, text, button, image, audio, video, group and row are all types of layers.
Layers are added to slides via the module editor. that is always present in every module, and is always visually behind all other layers.
A background can be transparent, a flat color, a gradient, an image, or a video. It can be animated in the same way as other layers can be, however it cannot be deleted.