I used to have a teacher who said this as well. The best way to understand would be to show examples of how to specify such terms as “stuff” and “things”. For instance: “The birds have lots of stuff.” That was not clear because the reader does not know what the “stuff” is. Instead, the text could be changed to: “The birds have lots of feathers.” Now the reader knows what the stuff is because it has been replaced with a more specific word. If you go over multiple examples in your head, it will become easier to comprehend and you will get used to doing it.
A. "Currently, there are around 900,000 pieces of debris between 1 centimeter and 10 centimeters in length orbiting in our space system, and this number is growing."
