Bleeds and Safety Margins
If you intend for an image to print all the way to the edge of the paper, it should extend 1/8" beyond the crop line. The extra 1/8" that will be trimmed away is known as the "bleed."
Bleeds are necessary because although modern trimming machines are capable of great precision, they are not perfect. If you create an image that is precisely the same size as the finished piece, and the press sheet shifts 1/64" while being cut, one side of the image will have 1/64" cut away, and the opposite side will have a white "flashline," 1/64" wide. By creating auxiliary image area that is intended to be cut away and discarded, you can avoid flashlines.