On the Editing page, the dark gray border around the book page represents the Trim area; any art that extend into this area will be trimmed off the page in production. Therefore, do not place any art (text, photos, art) in this area unless you want them to bleed off the page.
The inside, lighter gray border represents the Safe area. It is also recommended that you keep any important art out of this area, to prevent them from inadvertently being trimmed off the page.
The software will provide a warning if you place text in the safe area or trim area. You will not receive a warning for images placed in this area, as sometimes this is desired for design purposes.