Imagine you're out on a long run as part of marathon training or a long training ride on your bike in the process of preparing for a Gran Fondo. There isn't a lot of technical thinking going on, you're simply putting in the miles. When searching for motivation or where to put your thoughts you can let your mind wander aimlessly or you can consider the "why" behind the activity. The Why might be "I'm doing this long run to train my body to deal with 3 hours of running or 5 hours of cycling, this will help me when I feel I'm hitting the wall during my goal race." Or possibly it's a tedious workout in the gym that you've done a hundred times but you're doing it as part of your planned workouts. Your Why might be "I do this because consistency helps me attain my long term goal of lifting 200 lbs." It even applies to your diet when you are faced with yet another salad instead of that chocolate bar and beer you really want during training. "I'm eating a salad instead right now because the food I put in my body helps me attain my goals, reduce weight and give me energy for my next workout." Your "Why's" might be larger than those examples, they might include exercising because you want to live long enough and be healthy to see your kids grow, get married and have their own children. Maybe you have something to prove to yourself about your own fitness goals. The Why helps you to stay on task when the training or the goal becomes tedious. When you are tempted to cheat routine and ultimately cheat yourself.
Then there is the "What." Sometimes when I run I concentrate on certain form adjustments like my rear foot push off. I know that I have a quick cadence (which is great) but that if my toe push off is stronger I get a longer stride which helps me get free speed for the same energy output. When I concentrate on my back elbow pulling backward and my toe push off on my back foot my stride lengthens slightly while still landing mid or forefoot. That is a technique and requires concentration. When I swim I'll think about staying away from kayak swimming and work on a partial catch-up where my front arm doesn't start pulling in the water until my back arm nearly dips into the water. This is how my coach wants me to swim for longer distance with less energy, but it requires my concentration. I also work on looking at the bottom of the pool and keeping my face straight down, which in turn keeps my hips and bum up on the surface of the water rather than dragging below the surface creating drag. Again, that's a "What" in that I'm concentrating on a specific task with a technical purpose. The "What" is your mindset when your learning a task or concentrating on form that hasn't become completely easy for you yet.
Dividing my workouts (or even portions of my workouts) into "What" or "Why" has helped me retain focus and given me purpose. It's my biggest takeaway from the book and a very useful one. Now when I'm dragging my tired body out to the gym to work on my glutes I can realize I'm doing it because a strong bum helps me keep my old hip injury from returning (the "Why") and that motivates me to keep at it even when I'd rather just play a video game. When I'm out on a clinic run doing 450m repeats I can let my mind wander or think about driving my elbow back and pushing off with my back toe. That is my "What" and it not only helps me develop better habits it allows me to put my mind on something helpful rather than the pain of 10 fast repeats.
I hope this mindset is helpful to you as well. As Macy points out, this mindset is useful outside of fitness as well, whether it's in your career or personal life. When getting through a tedious day at work (maybe think about your why) or learning a new job skill (your what). It's a way of thinking that has helped me frame a purpose out of even the most mundane of tasks.