It's used to express happiness or approval for someone's good fortune, achievement, or positive situation.
Here are some examples:
* Friend: "I got accepted into my dream university!"
* You: "Que bueno por vos!" (That's great for you!)
* Coworker: "I just bought a new car!"
* You: "Que bueno por vos!" (Good for you!)
Note: The phrase "por vos" literally means "for you" in Spanish.