ZPL is the printer language used by Zebra thermal label printers. Instead of printing a picture of a label, the system sends the printer commands describing the text, boxes and barcodes, which the printer renders at its own resolution.
Printing native ZPL is faster and produces sharper barcodes than sending a PDF, which matters on a busy receiving dock. The label still has to be designed for the printer's resolution, 203, 300 or 600 dots per inch, or the barcode may not scan.
APFoodz prints labels to Zebra printers, or as PDF for any printer.