While you are editing your image, Ansel 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).