We don't know how you're defining it. Perhaps for the one image you uploaded, maybe you would say it's the background behind the floating cubes. For that image, thresholding alone is not enough to get it - you'd need edge detection also since some of the intensities in the cubes (non-background) are also the intensities present in parts of the background.
For other images it might be defined completely differently. There is no one definition that would be appropriate for all images in the world.
So you can't ask a general question like that and get a single answer. It's like asking in any random image I give you, identify the foreground and background. It's a matter of definition, which can sometimes require prior knowledge of more than just the image pixel values itself, like the geometry or whatever.
Best Answer