Each arm in the servo has a set number of joints (which you can see by the black dots on the lines) which bend the arm at those positions.
Turning on
Alternate Joint Direction will make each joint alternate which direction it bends in, whereas having it off will make all joints bend in the same direction.
Arm Offset Factor controls how each arm is oriented initially, so that instead of every arm beginning perfectly evenly around the servo, they will be offset by some factor, to create more intricate and unique patterns.
Clicking
Randomize Geometry Settings will randomize every setting, except the settings that are purely visual. Use this if you want a new set of settings with the same visual style.
Check out the screensaver! Every setting is randomized every 5 seconds, creating infinite unique designs.