An analogy for this, from the same book, is about paint.
Alice and Bob both start with a non-secret color of paint. Each one adds his or her secret color to the can, and sends the can to the other person. The only way Eve can determine what color has been added is to try a whole bunch of combinations. This is very hard. Once Alice and Bob have each other's cans, they add their own color. Since it doesn't matter in what order the paint is added, they both have the same color, which is used as the key. Eve can't see the final result, and adding the two colors as they are passed means she will have too much of the base color. so the result is secure.