The grey border you see in the editor is called our trim zone - it's .25" around all sides of your page/card that may be trimmed off during production.
*Note: This grey border is not present in the editor for products besides books. There, the trim area is automatically adjusted in each template, so how it is cropped in the editor is how each image will print
There is a small amount of variance in the trim area. The actual area that we trim is .125 inches around the edge, but since that area would be too tiny to see in our editor, the grey border technically denotes .25 inches of a trim area. This also provides extra assurance that even if something is accidentally dragged into the grey area, it might not be trimmed.
Essentially, here is what you need to know about this: If you want your photo to print full bleed, make sure that you drag the photo all the way to the edge of the grey border. If you do not want any of your photo trimmed, make sure that none of the photo is in the grey border.
^ Look out for this - this image may end up full bleed, or may end up with a small white border around it. Either pull the image in so none of it sits in the trim zone, or drag it to the very edge of the page: