What is "Resize original thumbnail"?

Can someone please explain to me EXACTLY what happens when I check “resize original thumbnail” in the Administration > Setttings > Thumbnails section?

When you change the setting in resize thumbnail the next images you import will be re sized to the new figures you put in the re size boxes. It will not change them there and then unless you go to global update /regenerate thumbnails, which I never do as I had problems with that way. When I want to change size I alter the size in admin/setting/thumbnails, as you said then I re import my product codes via csv with the images colum aswell.