Disable Generation
These settings determine which rounds of the terrain generation don't happen. Clicking an individual checkbox will disable that round, while clicking the "v" button to the right will disable or enable that round and every round following it.
Disabling large amounts of generation rounds can be useful for seeing what individual changes do, for example disabling everything from "Fill Continents" down is useful for modifying Continent Sculpting, while disabling everything and then incrementally enabling things can be useful as you adjust Terrain Generation Ratios.
Most of the rounds here are documented in either Continent Sculpting or Terrain Generation Ratios, with the exception of "Cleaning" and "Polishing" rounds which fix holes and otherwise smooth out the terrain.