The answer to this question can be found in the Prologue of Guns, Germs, and Steel. One of the formulations of the question is on p. 15 in the paperback edition of the book while the other is on p. 16.
In this book, Diamond is attempting to answer “Yali’s question.” This is, fundamentally, a question about inequality. Yali, a native of New Guinea, wanted to know why “white people” had so much “cargo” (material goods) while Yali’s own people did not. Diamond eventually wrote this book to answer the question.
On p. 15, he gives one formulation of the question. It is
Why did wealth and power become distributed as they now are, rather than in some other way?
In other words, why did Europeans become rich and powerful and able to conquer people in the Americas? Why wasn’t it the other way around?
On p. 16, Diamond gives the second formulation of the question. It is
Why did human development proceed at such different rates on different continents?
In this book, Diamond tries to answer these questions and comes to the conclusion that it was geographic luck that caused these things to be.