This generator works by overlaying a bunch of perlin noise-like patterns with different sizes to create some kind of fractal noise.
It was originally made for testing terrain and cave generation patterns for Volve.
Each layer uses 3 values: X for how wide it is, Y for how tall it is, and strength for how much effect it has on the image (smaller makes it have less effect). X and Y can have decimals, e.g. "0.5". Each layer is applied from top to bottom (bottom-most layer is applied last), so all layers except the first should have a strength below 1.
If you have trouble understanding it, try replacing all the text with this and fiddle around with the numbers: