The main page says:
But what is this? What is a “non-destructible, procedural graphic”?
In Microsoft Paint, if you change the dimensions of your image, then save it, the information of the previous dimensions of your image is lost. If you try to change it back, you might end up with a result that is different from the original picture. (destructive process)
In PixiEditor, if you apply a Scale node to your image, you can output the final result, but you can always access to the original image too. (non-destructive process)