Bleed
Bleed (also overfill) is the area in which the artwork is extended beyond the actual finished format. You set the background to run off the edge – into the seam allowance.
The reason: cutting and finishing can shift slightly. If the artwork reaches past the edge, no unprinted white “flashes” appear. Conversely, important content such as logos and text should stay a safe distance from the edge so that nothing is cut off or disappears into the seam.