One way to visualize the processing of an audio effect is to compare the amplitude of the output signal versus the amplitude of the input signal. This relationship is plotted on a two-dimensional axes, with the input amplitude on the horizontal (x-axis) and the output amplitude on the vertical (y-axis).
This is a different way to visualize the amplitude of a signal than the waveform.
For this plot, the idea is to display how the entire range of input amplitude values (e.g. from -1 to 1) get mapped to output amplitude values. For instance, if the input amplitude is 0.5, the plot will show what the output amplitude will be after processing.
This type of plot is helpful for understanding linear and non-linear processing. For linear operations (addition and multiplication), the slope and y-intercept of the line are changed. However, a straight line is always maintained. For non-linear processing, all types of curves, discontinuities, and piece-wise functions can be created.