Imagine a fish that has a white belly and a black back. When a fish looks up from below, the fish’s white belly looks like the bright sky, so the fish below can’t see it easily. When another fish looks down from above, the fish’s black back blends into the dark ocean water, making it hard to spot. This clever coloring helps the fish stay hidden and safe from other fish who might want to eat it.
Another way to say this is: The fish has two different colors that help it hide. Its light belly makes it look like the bright sky from underneath, and its dark back makes it look like the deep ocean from above. This helps the fish camouflage, or blend in, so other fish don’t notice it.