When photos, graphics, and text are placed too close to the edges or the gutter in the middle of the pages, you will see the caution borders and this message:
Warning - Your object is close to the edge/gutter and may be slightly trimmed during production.
To ensure that your objects will be fully printed, move them so that you do not get this warning.
You can set default margins to help with positioning of content on the pages. Check out this article: