The big question that Jared Diamond is trying to answer in Guns, Germs, and Steel comes from what is called “Yali’s Question" in the book.  It is found on p. 14, in the Prologue.  There, Yali (a native of New Guinea) asked Diamond

Why is it that you white people developed so much cargo and brought it to New Guinea, but we black people had little cargo of our own?

When Yali talks about “cargo,” he means wealth and technology.  In essence, he is asking why “white people” became rich and powerful and “black people” did not. 

That question stuck with Diamond and he eventually wrote Guns, Germs, and Steel to answer it.  By the time he came to write the book, the question had expanded somewhat.  On p. 16, Diamond gives a rephrased version of Yali’s question.  There, he says that he is trying to find an answer to the following question:

why did human development proceed at such different rates on different continents?

In Guns, Germs, and Steel, then, Diamond is trying to account for one of the fundamental facts of world history.  He is trying to answer the big question of why Europeans and their descendants have come to (at least for the past couple of centuries) dominate the world.

What is the underlying question for Guns, Germs and Steel?

The underlying question of this book is referred to as “Yali’s Question.”  It is a question that was asked of Jared Diamond by a New Guinean named Yali.  It is stated on p. 14 of the paperback edition of the book.  What Yali asked was

Why is it that you white people developed so much cargo and brought it to New Guinea, but we black people had little cargo of our own?

“Cargo” was the term that New Guineans used for all of the material goods that Europeans and people of European descent like Australians and Americans had.

The question, then, is essentially a question of why Europeans and their descendants became so rich and powerful.  By the time that Yali asked this question in the early 1970s, Europeans and their descendants completely dominated the world.  They were by far the richest and most powerful people in the world.  The underlying question of this book is the question of how this came to be.  Diamond spends the rest of the book trying to answer Yali’s question.