"Thanatopsis" starts by talking about nature's ability to make us feel better. The speaker tells us that nature can make pain less painful. It can even lighten our dark thoughts about death. He tells us that, when we start to worry about death, we should go outside and listen to the voice of nature. That voice reminds us that we will indeed vanish when we die and mix back into the earth.
The voice of nature also tells us that when we die, we won’t be alone. Every person who has ever lived is in the ground ("the great tomb of man") and everyone who is alive will be soon dead and in the ground too. This idea is meant to be comforting, and the poem ends by telling us to think of death like a happy, dream-filled sleep.