The answer to this can be found in the Prologue. On page 14 of the paperback edition, (the second page of the book), Diamond sets out Yali's question. It reads:
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 talked about "cargo," he was referring to material possessions. What Yali wanted to know was why white people had come to dominate the world to such a great degree. He wanted to know why they were so much more advanced and so much wealthier than the people of New Guinea (where Yali was from).
Diamond spends the rest of the book trying to answer this question and concludes that it was geographic luck that led to European domination of the world.