Editing is when you make changes, especially to a written document, that improve the finished product. A good English teacher will tell you that editing is just as important as writing.
Modifying your short story, cutting out some lines and adding others, is one example of editing. When a US senator makes changes to a complicated bill so it will please both Democrats and Republicans, it's also editing. Editing a work or document often makes it shorter, and it almost always makes it better. The Latin root of editing is editionem, "a bringing forth, producing," or "a statement or account."