The gyroscope of your iOS device is read out many times per second. All rotations of your iOS, in any direction and around any random axis, are correctly added and averaged over time.
The coloured gauge gives you a clear visual clue of current increase or decrease of rotation. The scale factor of the gauge is relative and is set by the sensitivity slider.
The number represent the absolute average rotation; it is not affected by the slider. This number can be compared to see trends in average rotation over time.
The sensitivity is automatically lowered if the gauge reaches its maximum position. You can therefore safely move set the slider to the far right and let the app determine the appropriate sensitivity for you.