Here's a breakdown:
* Cuando: When
* Vuelvas: The subjunctive form of "volver," meaning "to return"
So, it's used to express a future action or event that will happen after someone returns.
Examples:
* "Cuando vuelvas, te cuento todo." (When you return, I'll tell you everything.)
* "Llámame cuando vuelvas a casa." (Call me when you get home.)
Let me know if you have any other questions!