To get the best answers, it would be better to ask a more specific question. As it is, all we can do is to try to explain the basic thrust of Chapter 8 without knowing what parts are difficult for you.
Diamond’s main argument in the book as a whole is that geography is what decides who “won” and “lost” in human history up to around 1500 AD. Many people think that the racial or cultural characteristics of various people are what decides who wins and loses. Diamond disagrees and says that it is just geographical luck.
In Chapter 8, he argues that geographical luck caused agriculture to develop in the Fertile Crescent sooner than it did in New Guinea or in North America. He says that the main factor here was the number of domesticable plants and animals that were available in each area. Diamond points out that New Guinea and North America did not have many plants that could be domesticated. By contrast, the Fertile Crescent had three different important kinds of grain, four kinds of legumes, and flax, which could be used to make cloth. Because the Fertile Crescent had “better” plants and animals, agriculture could start sooner there than in New Guinea and North America. This gave the people of Eurasia a massive head start over people in other parts of the world.