A Julia set: one of the classic fractal shapes, and a good place to get lost for an hour.
The rule behind it is tiny. Take a pixel's position, feed it through the same short sum over and over, and watch what happens. Some starting points settle down and stay put no matter how long you run it. Others shoot off to infinity. Colour each pixel by which fate it met, and how quickly, and the boundary between the two turns out to be infinitely detailed: zoom in anywhere on it and there is always more structure.
cReal and cImag set the constant in that sum, and changing them changes the shape entirely, so the poi list of preset values is the quickest way to find the famous ones. The output modes are five different things you can say about each pixel's journey rather than five different fractals: how long it survived, how close it came to the edge of the set, or how near it passed to a chosen shape along the way.