So I've always been wondering what "뭐 해" meant when I listened to"Rhythm Ta" I've always had an idea and finally looked it up and I was right 😁.
뭐 해: what are you doing?
여기서 뭐해?: What are you doing here?
기금 뭐해?: what are you doing right now?
I believe to make it formal you add "요" at the end. I hope this helps some of you. Please feel free to correct me if I'm wrong.