You can think of destructive workflows as any change that makes the work in a previous step no longer relevant, up to date, or editable.
The best analogy I can think of is Photoshop Layers. Without using them, every edit you make to an image is potentially destructive to the last step.
Now think about asset production for a game or film, such as a character. Multiple departments (modeling, rigging, sim, etc) potentially have their own edits to make. In a traditional destructive workflow, a change to the base asset will require all subsequent edits to be re-done. In a non-destructive pipeline each department is free to make changes without clobbering the work of others.
Keep in mind that certain edits will still require changes downstream, but fixing the problem may no longer require re-doing the work from scratch.