This is the 2537th most frequent Korean word.
뭘
"뭘" means "what" or "something" in casual contexts.
The word '뭘' is a contraction of '무엇을' which is the object form of 'what'. It is used here inquiring about the subject's object of search in a formal polite manner.
뭘 찾고 있어요?
What are you looking for?
Here, '뭘' takes on the nuance of 'why' or 'for what reason' in an interrogative sentence, questioning the cause of the subject's reaction.
뭘 그렇게 놀라요?
Why are you so surprised?
In this sentence, '뭘' resumes its role as an object interrogative pronoun, asking about what the subject prefers in a polite formal register.