It is not really possible to summarize Yali’s question as Yali’s question is made up of one short sentence. Yali’s question is given in Guns, Germs, and Steel as
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?
There is not really any room to summarize this question. Instead, let us explain it.
What Yali is asking is why some people of the world came to be wealthier and more powerful than others. He had seen how much in the way of material goods (this is what he calls “cargo”) the white people who came to New Guinea had. He had seen that they had so much more than his own people in New Guinea. Therefore, he came to wonder why that state of affairs existed.
Yali’s question, asked in 1972, became the core of this book that Diamond wrote many years later. Diamond is trying to explain why Europeans came to dominate the world. He is trying to answer Yali’s question in Guns, Germs, and Steel.