What is the meaning of narrow down scope questions?

Limit a question so that it is possible to answer it. For example, a question like "Who were the most popular stars?" is impossible to answer. The scope of the question--the field, the breadth of the topic--is too large.

You could narrow it by adding specifics and qualifiers; for example, "Who were the most popular Hollywood movie stars in the 1940s?" or "Who were the most popular stars of prime time television in the U.S. in 2007?"