Crop marks (also called trim marks) are small guide lines placed at the corners of a print file, showing a printer exactly where the final piece should be trimmed to its finished size.
They work alongside bleed — bleed provides the extra margin to account for slight cutting variance, while crop marks indicate precisely where that trim should land.
Crop marks are typically added automatically by design software when exporting a print-ready file, but it's worth confirming they're included (and set correctly) before sending artwork to a printer, especially for a custom-sized piece.