If you guessed clouds, you are right! The image above (and at the bottom of the page) shows clouds on Jupiter (Image Credit:NASA/JPL-Caltech/SwRI/MSSS/Gerald Eichstadt), but these are not your typical Earth-clouds. On Earth clouds can only grow so tall, but on Jupiter that appears to not be the case.
Because of the way our atmosphere works, clouds can only rise so far then they basically hit a ceiling. The image below from NASA shows this happening. The top of the cloud is hitting the next layer of the atmosphere, but it can't go there. It literally acts like it hit a ceiling, and since it can't go up any higher, it spreads out.
On Jupiter it seems clouds are not flat on top. The bright patch on the right side of the image are the high-altitude clouds, but notice how each cloud top looks a little different. Some areas are brighter than others, some look sharper than others, and some are more spread out. This inconsistency shows they are not all the same, so they are not flattening out on top like they hit a ceiling.
The Juno spacecraft was about 6000 miles above the tops of these clouds when it took this image.