A dieline is an electronic file usually supplied by the printer to show where the measurements and the cut marks are for a specific print or package. Think of this like a blueprint of your print job before your order is sent to be manufactured.
Dielines need to be created in 'Adobe Illustrator' and you need to ensure that the colors used are in CMYK format. When sending Adobe Illustrator dieline files to a supplier, check that all images are embedded so that there aren’t any missing images when the supplier receives the file.
Here's an example of a dieline for a folding carton box: