Glossary
Mockup
A mockup is a rendered image showing your artwork applied to a product, used in listings before any physical item exists. It is a preview, not a photograph of the item a buyer will receive.
Mockups do most of the selling in print on demand, because the product genuinely does not exist until someone orders it. A flat artwork file converts far worse than the same design shown on the actual item in a real setting.
Two practical cautions. A mockup must reflect the real print area and placement, or buyers will feel misled when the item arrives. And a generic mockup that every seller in your niche is also using makes your listing look interchangeable, which is a real disadvantage on a marketplace that shows dozens of similar results side by side.