It's a common French phrase used to ask someone about their occupation or profession. It's a general inquiry about how they spend their time and what their purpose is in life.
Here are some other ways to ask the same question in French:
* Quel est votre métier? (What is your profession?) - This is a more formal way to ask.
* Qu'est-ce que vous faites pour vivre? (What do you do for a living?) - This focuses on the work someone does to earn money.
* Quel est votre travail? (What is your job?) - This is a less formal way to ask about someone's occupation.
The specific translation of "ce que tu fais dans la vie" might vary depending on the context, but the meaning is always related to someone's occupation or purpose in life.