What is the white box on the back of my KDP cover wrap?

It is the place where KDP prints the book's barcode. KDP prints a barcode of 2 × 1.2 inches in the bottom corner of the back cover, near the spine, at least 0.25 inch from the spine and from the edges, and requires that area to be solid white: a cover with text or pictures there is rejected. From version 1.9.20 the print wrap in the Covers page keeps exactly that area clear, and lays out the back cover text around it. "Keep KDP's barcode area clear" is on by default; untick it only if you print your own barcode somewhere else on the cover.