The only way to briefly summarize this book is to say that geography is responsible for nearly all the differences in human technological and political development.
Diamond's main argument is that geographic chance (presence of domesticable crops and animals, length of east-west axes, etc) is what made some places more likely to invent agriculture and to develop technology. The places that did these things became the "haves" of the world while those that did not became the "have-nots." Diamond emphasizes that this happened because of geography and not because of anything that was inherent in the nature of the people or in their cultural values.
So, the summary is that geography (and not racial or cultural superiority) causes some societies to become "advanced" while others fail to advance.