The natural world is only a facsimile of Heaven, the desert is important because by immersing itself in it you can learn about the existence of the world, and a single grain of sand symbolizes the marvels of creation.
In The Alchemist, a shepherd boy goes on a quest to find himself. Part of his spiritual journey involves teaching by the mysterious alchemist, who shows him the way of life.
The alchemist tells the shepherd boy that the natural world is only a copy. He shows him the Emerald Tablet, which leads to an understanding of the Soul of the World.
"The wise men understood that this natural world is only an image and a copy of paradise. The existence of this world is simply a guarantee that there exists a world that is perfect….” (Part 2)
Nature represents Heaven, but is only a copy. We can never see all of nature, or understand all of the wonders of the world, as long as we are here on Earth. Our only hope is to develop an understanding of nature.
As they are traveling in the desert, the alchemist uses it as a metaphor for the understanding of the greater world. The desert is so immense, the alchemist tells the boy, that by immersing yourself in it you can actually study the meaning of life. Careful study of any part of the natural world will bring this understanding.
“… You don't even have to understand the desert: all you have to do is contemplate a simple grain of sand, and you will see in it all the marvels of creation." (Part 2)
Since a single grain of sand can symbolize “all the marvels of creation,” we can find big ideas in even the tiniest forces of nature. By analyzing the small, we understand the large. Zooming in on a grain of sand shows us the big picture in a small package.
Each of these lessons are designed to teach the boy, and the reader, to look more closely at the simple things in life. The meaning of life does not come from the big things, it comes from the small things. By focusing on these, we can reach enlightenment.