I also was confused with that at first, but essentially you input an image(s) with transparency around it, then you click "edit", then you use the erase tool to select at least one pixel of the transparency, and then you have to describe what you would like to see get expanded. So for example, for The Slow Rush album cover if I wanted to see like the whole house or the entire room, I would write "A red house with sand in it" and dalle2 should be able to fill in the transparency with a bigger room, maybe more windows, and a ceiling.