What are crop marks?

Crop marks are lines in the corners of the papers that show how to cut away the margins.

If you do not add a margin, the crop marks cannot be displayed (as there just is not any room for them).

Note that if you select the Overlap option, the crop marks are positioned at the non-overlapped part of the image, and cutting by them will actually seem like you are cutting away part of the image (but it is duplicate of the part of the next page inside the crop marks).