A higher stack for a given reach puts you in a more upright riding position; your hands will be moved upwards, and your body position will be more vertical. If you keep the stack the same but increase the reach, you'll have a longer riding posture; your hands will go forward, and you'll be more spread out. You can fine-tune your ride position by changing your stem, adding or removing headset spacers, and so on, but only so much, which is why stack and reach are so crucial to a bike.