I learned recently that a mock-up is a downloadable image of a blank t-shirt (or sweatshirt, tank, etc.) that allows you to add your design on top of the image, to give you and your customers an idea of what the finished product will look like. It is important that a good mock-up should look no different than the actual finished product. I learned about it when I went with one of my daughters to get a T-shirt from https://www.zaynmalikmerch.shop. She's been a fan for a while, but I got to learn something new that day.