"What did you do for nine days?"

Translation: 아흐레 동안 뭐 했어요?

[Philkyriak]

What does "동안" mean here?

[Catsy_Brave]

For a period of time / within a period of time.
So the sentence says: Nine days within what did?
You would use 동안 in sentences like: "I have learnt Korean for 9 months."

[RQZ.Sash]

What is the difference between 뭐 and 무엇을 and how should I use them apart?