Oh, that! These buttons make your workout a little easier or a little harder. You see, Zwift stores all workouts in percents of FTP, and scales them for you based on your personal FTP number. If you start a workout and feel full of energy, you can make it a little harder, by up to 10%, by effectively increasing your FTP just for the duration of the workout, or maybe even just for a single interval. If you feel tired and you are struggling with generating the power required to complete the workout, you can make the workout by up to 10% easier, by effectively reducing your FTP temporarily. This is what these buttons do. If you press the “-” button, you will see that target power for all intervals of your workout (as displayed on the left-hand side of the screen) will scale down and pedaling will become a little easier. “+” button will scale all numbers up, and workout will become a little harder. You can always change it back and forth, if your feel you need to, as you move through the workout. It would not permanently change your FTP, it is an adjustment for just the workout which you are riding. This adjustment will disappear once you complete the workout or the ride, and your FTP will return to whichever number you have stored in the system.
You can make these adjustments in, if I remember correctly, 1% increments. It is convenient to do on the companion app screen, if computer keyboard is too far from you.
If at any time you decide that the FTP number determined from the test consistently feels too high or too low for you, you can take another test, or simply adjust the FTP manually in Zwift. For example, when you pick a workout, your FTP will appear in the bottom of the window, you can click on it and edit it as you wish. This will be a “permanent” change, until you do another FTP test or edit it again.