Simply move the sliders around, and try the check boxes. Amplitude controls the lobe sizes. Phase controls lobe orientations. Spiro gangs the x and y amplitude and frequency controls, and sets the phases for symmetric spirographs. Decay makes the curve spiral inwards.
Color sets a black background, and draws the picture in color.
Thick draws a thick line. This also slows the drawing speed. Leave the phases alone for true spirographs. You can change them for interesting effects, but the pictures won't then generally be spirographs.