In this chart, Diamond is attempting to show us what factors led to the Europeans' possession of "guns, germs and steel" and other "proximate causes" of their ability to defeat people such as the Incas.
At the bottom of the chart, we see the things that actually allowed the Europeans to be so dominant. These are things like horses, guns, ships, political organization, and epidemic diseases. We all know that these things helped the Europeans, but Diamond says knowing this is not enough. Instead, he says, we have to understand why the Europeans had these things while the Incas and others did not. This is the purpose of this chart.
The chart shows the factors that made these proximate causes happen. For example, we can see that Europeans had epidemic diseases because they had many domesticated plant and animal species. We can see that they had many domesticated species because of things like the east-west axis of Eurasia and the consequent ease of species spreading.
Overall, then, this chart shows us the ultimate causes of the things (horses, guns, diseases, etc) that allowed the Europeans to dominate the world.