Yali's question is why Europeans have so many more material goods than other peoples. This is caused in part by the fact that the Europeans were able to conquer other peoples and exploit their resources. This ability was caused in large part by the fact that Europeans had "germs."
Many infectious diseases had evolved in Eurasia because of the fact that large animals had been domesticated there for a long time. Eurasians developed resistance to these diseases, but peoples from other continents did not. This meant that those people were killed in large numbers by European diseases when they were exposed to those diseases.
The germs helped the Europeans conquer and this helped give them more resources than people from other continents had. This is how germs help to answer Yali's question.