When Poe discusses the idea of nature, he holds it in high reverence but also acknowledges its mysterious and frightening qualities. Poe was fascinated with the macabre, so clearly the terrifying aspects of nature would have piqued his interest.
The first thing Poe says is that nature is sacred, and that idea was shared in common by many writers of his day. Nature was seen as an unspoiled piece of this world which should be held in high regard and protected from humanity. Many people, during the 1800s and early 1900s just as today, believed that humanity's actions, especially increased industrialization, were inherently evil and destructive to the earth. Because of this, they argued humanity was squandering a precious natural resource and destroying the earth when it polluted and took over large areas of land.
The main portion of the quote, however, says that nature is dark and mysterious and holds the secrets to life and death. Nature is in a constant cycle of change—moving from life to death frequently. Just observing the passing of the seasons shows birth, life, old age, and death every year. Also in nature, animals are killed and eaten, and they die seemingly without cause or purpose all the time, but there is no evil in these actions.
I believe one of the things Poe is referencing is that the cycle of life and death is not an inherently bad or mournful, or even evil, thing. In fact, it seems to all be a part of the way the world works, and it should therefore be accepted as natural, even though it is difficult to accept death as a part of the cycle of life.
By speaking to this, Poe also notes that nature carries the secret to life. Acceptance and appreciation are vital to living a healthy and joyful life. By accepting the cycle of life and death, and also by respecting the world around us—through not spoiling the land, caring for others, enjoying nature, and more—we can live a healthy and fulfilling life.