The professor fills an empty jar with stones to represent important things like family and health. He then adds rice representing less important things like work and possessions. Finally, he adds sand to fill the remaining spaces, representing small unimportant things. He explains that if you fill your life with unimportant sand first, you won't have room for the more important stones and rice. The lesson is to prioritize the most important people and aspects of your life to find happiness and avoid getting caught up in unimportant small things.