Crop marks and bleed
Crop marks, also known as trim marks, are lines printed in the corners of your artwork layout or sheets of paper to show the printer where to trim the paper. They are used by commercial printers for creating bleeds where an image or color on the page needs to extend all the way to the edge of the paper. The space from crop marks to trim size is 3mm. So, the printed paper with crop marks are larger than trim size.
Crop marks are crucial if the artwork layout with bleed.
The bleed is the area to be trimmed off. Artwork and background colors will extended into the bleed area. In common, the bleed area is 3mm or 0.125″. For mass production, it is difficult to print and trim exactly to the edge of a sheet of paper. So, the bleed looks like the tolerance for binding and trimming which can achieve. After trimming, the bleed ensures that the ink prints to the edge and does not stop short of the color or background.