There are a number of places in this book where you can find Diamond discussing the difference between proximate and ultimate factors.
The first place where Diamond gives a useful definition (or a hint at one) is in the Prologue. On page 23 in the paperback edition, Diamond tells us that proximate causes are those that identify "immediate causes." Still in the Prologue, on page 28, Diamond tells us that ultimate causes are the ones that lead to the proximate causes.
The best place to see a graphic illustration of the difference between these types of factors is in Figure 4.1, on page 87. There, Diamond gives us a flow chart that shows how ultimate factors led to the proximate factors that made European countries stronger than those of the Americas and other such places.
While there are many other mentions of proximate and ultimate causes, these three should serve to illustrate the meaning of these concepts.