Answer
Only more parallel rays hit the center part of the lens.
Work Step by Step
At a larger f-number, the opening in the iris diaphragm is smaller. Only light rays hitting the central part of the lens are used to make the image, and these rays are more parallel than if a larger opening (smaller f-number) were used.
These more parallel rays create smaller circles of confusion (as discussed on page 716).
Objects over a greater range of distances will be in focus.
