Crop marks indicate where the cover or pages end and where the paper is cut to your specified book size.
Note: The safe text area is inset from the crop marks by 18 pts (.25 in, 0.6 cm) on all outside edges and 45 pts (.625 in, 1.587 cm) for the gutter. This indicates where you can insert text and images and be sure that they will not be trimmed too close to the edge or bound into the gutter. Any and all critical content, like text, should be kept within the safe area. It is similar to defining a margin in word processing documents.