Spiro recreates the famous 60's toy Spirograph, in which geared rings and wheels were used to generate variaties of curves from a family known as hypotrochoids. The parameters are self-explanatory: a moving circle (moving radius) with a pen at a distance of pen offset from its centre moves inside a circle of fixed radius, tracing out a graceful curve.
Advanced spirograph users will remember that by using different holes in the wheels, and by creating various gadgets with the rings and the bar pieces to move and rotate the fixed ring, you can build up complex and beautiful designs. Use the how many value to draw a curve a number of times, and the various change values to alter characteristics of the curve on each pass. Selecting one or more of the unwind options will continue the pattern but step the change for that particular parameter back to its original value - try it! Transparency - of course - controls the transparency of pens used, so underlying lines won't be hidden by new drawing. The checkbox apply dx/dy to rotated controls the way the transformations are applied - ordinarily every change in x and y is relative to the paper's implied grid, if this is checked the changes are applied to the grid as currently rotated. Try it and see what difference it makes.
Software, of course, lets you do things that were physically impossible with the original plastic set. Try making the moving radius larger than the fixed one, or putting the pen beyond the radius of the moving wheel. What's more, the paper will never slip, the pens won't tear the paper, and you'll never see the pin holes. Experiment and enjoy!