The 'Saturation Amount' control applies boost to the signal as it passes through the saturation effect, thus increasing its tendency to saturate.
The 'Saturation Shape' control affects the nature of the waveshaping applied to the signal. At a value of 0.0, the effect is simple hard clipping. At 1.0, the effect is soft clipping. Beyond 1.0 the effect is increasingly more extreme waveshaping.