What do the red and yellow borders in Step 2 of the editor mean?

If you add an image that you want to bleed to the edge, be sure to drag it into the red area so that it does not leave any unintended spacing between your image and the edge. Also be sure you don’t have any text or other important content that is near the edge, so it does not get cut off or print too close to the edge.

The inner yellow border indicates the margin that you should keep text within. If you put text into the yellow border it will either get cut off or will be too close to the edge.