In short, Schopenhauer is saying that the world is uglier (and more ferocious) than it appears. While nature may seem to possess a tranquil beauty to the human observer, we miss the underlying savagery that characterizes the world. Thus our idealization of nature, and any optimism about the world we try to build on it, is ultimately an absurd fantasy born of self-deception.
Here is the context:
“This world is the battle-ground of tormented and agonized beings who continue to exist only by each devouring the other. Therefore, every beast of prey in it is the living grave of thousands of others, and its self-maintenance is a chain of torturing deaths. Then in this world the capacity to feel pain increases with knowledge, and therefore reaches its highest degree in man, a degree that is the higher the more intelligent the man. To this world the attempt has been made to adapt the system of optimism, and to demonstrate to us that it is the best of all possible worlds. The absurdity is glaring. However, an optimist tells me to open my eyes and to look at the world and see how beautiful it is in the sunshine with its mountains, valleys, rivers, plants, animals, and so on. But is the world, then, a peep-show? These things are certainly beautiful to Behold, but to Be them is something quite different.”
(Arthur Schopenhauer, The World as Will and Representation, Ch. XLVI)