Crop marks
Crop marks are short lines printed at each corner of a card (just outside the trim line) that show the printer exactly where to cut. They sit in the bleed area and are trimmed away with the rest of it.
Why it matters
Home-printing on letter paper is the most common reason to need crop marks — you cut cards with a guillotine or paper trimmer and the marks tell you where to slice. Pro printers add their own marks, so a CMYK PDF for a print shop usually skips them.
Example
A PDF of nine poker cards on a letter-size sheet has tiny "L"-shaped marks at every card corner. Line a ruler up to the marks, cut, and you get clean 2.5" × 3.5" cards.