Whether you agree with Diamond or not, his theory is interesting in providing a likely explanation of the development of our species. It is what we now call 'big history' (the big picture), rather than traditional history which concentrates on documented sources.
As other's have pointed out, Diamond theorizes that human civilization is strongly a product of geography. In this way, Diamond's ideas can be said to be deterministic. For example, topography and climate will guide (or separate) human groups, either aiding or limiting the spread of ideas and developments.
The argument goes that Eurasia with its open plains and east-west orientation permitted the spread of people's and agriculture. By contrast, the other continents with their north-south orientation meant that topographical barriers (Rockies, Andes) and climatic bands (Amazon Rainforest, Sahara) limited the spread and diffusion of migration and therefore ideas (and of course microbes!).
I hope that helps enough for you to understand Diamond's thesis a little better.