Countershading is a type of coloration commonly found in animals, and means that the animal's back (dorsal side) is dark while its underside (ventral side) is light. This shading helps an animal blend in with its surroundings.
In the ocean, countershading camouflages an animal from predators or prey. When viewed from below, an animal's lighter belly would blend in with the lighter sky above. When viewed from above, its darker back would blend in with the ocean bottom underneath.