In many cases when you upload a product to the admin of your site, you will include more than one image. Perhaps an isometric shot, as well as a front-on view. 

Websites need to know what order to display those images in because the primary image will show up in your main catalog, and that same image will show up first when someone clicks on the product. 

So when you upload a new product to the admin, whichever item is leftmost will be the primary image shown everywhere. And all you need to do to reorder an image is to drag and drop it in the position you want, and hit "Save". Nice and simple.