What is an Overarching question?

It is a question that presents an "issue". An issue is a dispute disguised as a question.

A good example would be, "What should be done to fix the economy?" There is no easy answer and it invites a dispute or debate in order to find a solution. Therefore, that would be an example of an "overarching question" because the issue created by the question is much larger than a simple answer and invites debate.

A simple question would be, "Is the sky blue?" That's either a yes or no question and unless someone wants to mince words, there won't be much debate about it.

An overarching question would be, "How high is up?" which begs an explanation and debate as to where the sky actually ends and what remains beyond it. Theoretically it could be infinitely high, or there may be some limitation and in order to avoid an arbitrary conclusion, debate will be engaged to determine where "high" and "up" are in order to determine or measure that distance.