Traditional photo editing is ‘destructive’. That means every adjustment you make permanently changes the pixels in the photo and there’s no way back unless you’ve saved a copy of the original and you’re willing to start again. ‘Non-destructive’ editing is fully reversible. You can go back and undo or redo all of your editing work at any point in the future. Naturally, there’s a catch
Many programs can store a ‘history’ of all the editing changes you’ve made since you opened an image. Using this you can check what you’ve done and even backtrack to an earlier image state if you realise you’ve made a mistake. Some programs can store the history as part of the saved image file, while non-destructive editors like Lightroom will store it indefinitely as part of the image’s adjustment metadata.