What is a opaque fish tank divider?

It is exactly what it sounds like... it's basically a wall that separates your aquarium into sections so you can keep incompatible fish in the same aquarium; or if you're breeding fish that could be dangerous to each other (like flowerhorns, trimacs, bettas, etc.), you can use a divider to separate them until you are ready to let them mate, to reduce the risk of injury/fatality. And "opaque," just means that it is not see-through, so the fish will not get stressed from an endless war over dominance.