Short form of background. What lies behind the attended thing and conditions how it reads.
Clipped from background: Old English bæc (the rear, the spine) + grund (the ground, the bottom of a thing); the compound narrowed in painting to the distant part of a scene before it named whatever lies behind and conditions a foreground.
What sits behind the attended thing and quietly sets the terms for how it reads, shortened to the two letters a file name has room for.