This is the 14412th most frequent Korean word.
무엇일까요
"What could it be?"
Here, '무엇일까요' is used to express curiosity about an unknown object.
이게 도대체 무엇일까요?
What could this possibly be?
In this context, '무엇일까요' is used to prompt for an explanation or reasoning.
내일 날씨가 어떨지 궁금해요. 비가 올 수도 있으니 우산을 챙기는 게 좋겠죠. 무엇일까요? 우산을 챙겨야 할 이유입니다.
What could it be? It's the reason to take an umbrella.
This usage of '무엇일까요' seeks to understand or inquire into a motive or cause.