In Midjourney, U1 refers to the first upscale command option that can be applied to an image generated by the AI model. When you click the U1 button on an image in Midjourney Discord, the resulting image is a crop of the initial generation of the image. Each of the four upscale command options (U1, U2, U3, U4) corresponds to one of the four images in the initial generation grid, going clockwise starting with the top left image. The upscale options allow you to increase the resolution and size of the image, with each option providing a different level of detail and alteration to the original image.