The old woman tells Santiago that dreams are the language of God.
Santiago is a shepherd on a mission. He is looking for the Language of the World. He had a dream that he needed to look for treasure, so he went to a gypsy palm reader. The old woman tells him she can interpret dreams, but with a caveat.
When he speaks in our language, I can interpret what he has said. But if he speaks in the language of the soul, it is only you who can understand. (1)
Santiago tells her that he has dreamed of a boy who took him to the Egyptian pyramids to find a "hidden treasure." She tells him that he does not need to pay her, but needs to give her part of the treasure when he finds it. She also tells him that his dream is complicated and he is not wise enough to understand it.
This is one of the reasons Santiago begins his search for his personal legend. He is a wanderer and a thinker. Eventually he realizes that what he sought was back where he began, but he learns a lot about himself along the way.