Read it. It's quite digestible and informative. Here's my quick synopsis:
Diamond is an evolutionary ornithologist who spends a lot of time in New Guinea. An indigenous New Guinean man asks him one day why Diamond's people, meaning people of European ancestry, have so much cargo (stuff), while his people (and by extension the rest of the colonized or formerly colonized world) have so little. He thinks about it for twenty years, and this book is his attempt to answer the question. In a very simplistic form, his answer is this: geography. Eurasia had the good luck to have an east-west orientation in which there is less climatic variance than is found on north-south oriented continents (the Americas and Africa). Additionally, Eurasia possessed abundant natural resources which included plants that were both nutritious and domesticable, large mammals that were domesticable, and the minerals that could be manipulated into metal tools and weapons. Plants that could be domesticated and were nutritionally rich enough to supplant hunter/gatherer lifestyles led to settled and semi-settled populations and a food surplus. A food surplus frees members of said society from the otherwise constant duty of feeding themselves and allows them to become specialists in other things, which leads to innovation (in things like metal tools, weapons, and writing). East-west orientation means that new knowledge in farming is easy to transmit over a landmass because the climate is more constant and in Eurasia there are fewer geographical barriers (like the Andes or the Isthmus of Panama) to inhibit the flow of information and technology. Settled populations with access to domesticable mammals learn to use those animals and therefore live in close proximity to them, which leads to the germs that infect those animals eventually learning to infect humans too (which is where we get the flu and all of the other modern and historical epidemics). Those humans develop resistance to those diseases. So by the premodern period we have cultures in Eurasia (like the Spanish, English, Dutch, and French, among others) who have the good fortune of centuries, millenia, of information passed along to the from the trials and errors of their ancestors, writing to pass information along with, a host of diseases and resistance to many of them, metals that have been shaped into swords and guns and armor, domesticated horses to ride into and out of battle or to carry information quickly over long distances, and food enough to supply (with varying degrees of success) long open ocean voyages of discovery. Transport those people to other parts of the world, where large mammals succumbed to quick extinction upon the arrival of humans that they hadn't evolved to fear and thus were easy dinners, and were therefore without animal borne disease or resistance to it. Diamond estimates that 95% of the pre-1492 population of the New World was killed by disease before they every saw a white man. They had much less advanced metallurgical technology with which to defend themselves, and thus the survivors of the sicknesses had to contend with guns and steel swords and armor with wood and bone weapons. This is why the Incans were subdued by the Spanish rather than the other way around. Diamond expressly dismisses any notion of racial superiority. He points out that people, the world over, have become adept at surviving and even thriving in the geographical and natural situations in which they have developed. Most of us would die if we were suddenly dropped in the Congo or New Guinean rain forests, where those respective peoples have managed to live for thousands of years.
So that's the meat of it. I just reduced a four hundred page book to a few hundred words, so of course you can poke holes in it. But Diamond makes a good case for his theory while recognizing that he is, himself, spending 400 pages on the whole of human history, and thus necessarily simplifies. On the whole, though, I'd say he's got it right. In any case, you'll learn a lot about the world by reading it.