It is used to express:
* Surprise or disbelief: "Comment ça tu as raté l'examen?" (How did you fail the exam?)
* Confusion or lack of understanding: "Comment ça tu ne comprends pas?" (How do you not understand?)
* Request for clarification: "Comment ça tu vas partir en vacances?" (How come you are going on vacation?)
It's important to note that the meaning of "comment ça" can vary depending on the context and the tone of voice used.