undo and redo
While you are editing your image, darktable records all of the modifications you make to that image. This means that it is possible to undo and redo changes to recover a previous editing state. Note that the undo/redo facility is unlimited in the number of steps while editing an image, but is reset each time the darkroom is switched to a new image.
Press Ctrl+Z to undo the last modification and Ctrl+Y to redo the last undone modification (if any).