The snake symbolizes death, in exactly the same way that the snake symbolizes death in the Garden of Eden. One of the dominant themes in The Little Prince is experiences beyond the corporeal. The little prince comes from a very tiny planet that can't really be seen from earth. The invisibility of his world represents spirituality. There are spiritual things that cannot be seen and are of greater importance than anything corporeal that can be seen, such as love, friendship, faith, peace and harmony. The prince uses the snake to return home to his planet because, as he says, "It is too far. I cannot carry this body with me. It is too heavy" (Ch. 26). Hence, the little prince's ascension back up to his planet with the use of the snake represents a spiritual ascension and a return to the things that are of greatest importance, such as love. Hence, the snake represents death, or a release from the corporeal world into the spiritual world.
The little prince first learns from the snake to look beyond the physical realm when he first meets the snake. The snake points out that even though he is "no thicker than a finger" he is "more powerful than the finger of a king," which is due to his ability to transform the corporeal world into the spiritual world through death (Ch. 17). Hence, the prince learns from the snake, which symbolizes death, that in order to return to the things that truly matter, he must have a spiritual ascension.