The first premise to understand is that simple harmonic motion through time generates sinusoidal motion.
The following diagram will display the amplitude of the harmonic motion and for this we need to use the term A in our formula. We will also be using θ.
I have used the equation A sin θ where θ completes one cycle (degrees).
The y axis displays values based on a unit circle with A being interpreted as amplitude.
The x axis denotes degrees (θ)