As Instructor, while you are in your test unit and creating your questions you will notice under the answer options the Tags feature (1).
You can create a new tag by typing the word that you want to use (2) and add it to the list (3).
When you go back to the list of questions the tag will appear in a small grey box (4).
Clicking on the tag will lead you to the list of questions that contain this tag (5).
A question can have more than one tag, and clicking on a specific one you will get a display of all the questions that contain it.