The phrase "folder background" refers to the image or color that appears behind the icons and files in a folder on a computer. It is the visual appearance or design of the background within a folder. Full definition
One PDF or ePub file is dropped into / documents folder background conversion process is started automatically and after some time a converted document appears in it's place. (ereadertech.com)
While Action Launcher can have separate colors for the icon and actual folder background, Nova Launcher makes you use the same hue for both. (androidcentral.com)
There are two versions available for theme addicts: a premium $ 0.99 edition that offers additional customization such as their signature icon set, skinned media player and folder backgrounds, dubstep wallpapers and a limited feature free version with OS 7 icons. (crackberry.com)