A person's raison d'etre is their purpose or reason for living. It's why they do things.
In French, raison d'etre literally means "reason for being," and in English it means about the same. Someone's raison d'etre is the most important thing to them: Michael Jordan's raison d'etre was basketball. Beethoven's raison d'etre was music. A raison d'etre motivates a person; it's their purpose. Taking care of your family could be your raison d'etre, and so could your career. Raison d'etre is a strong term for something that gives a person's life meaning.