What is a Personal Legend, and why do you think this will be important in the rest of the book?

In The Alchemist, the concept of a "Personal Legend" is explained by the King of Salem as "...what you have always wanted to accomplish. Everyone, when they are young, knows what their personal legend is." He goes on to say more about how the Personal Legend stays bright in a child's mind until a "mysterious force" convinces them that their goal isn't possible. The pursuit of a Personal Legend will surely become more important in the story, seeing as Santiago (the boy) is torn between traveling and going after marrying the merchant's daughter. The legend will serve as an impetus to start him on his journey towards fulfillment.