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
If you shoot RAW files rather than JPEGs and then use a tool like Lightroom or Adobe Camera Raw to edit them, they will need to create a 'sidecar' file alongside the image to hold the editing data, since RAW images cannot be edited or modified.
Not all programs use sidecar files. Some will create a new folder within the image folder to hold the processing metadata, and others will store this in image catalogs or databases.