"Editor notes" are the little invisible notes that appear within the brackets. To insert an "editor note" into some text, look up at the top of the editing page, there's a row of buttons there. Find the one that looks like a blurb balloon. Mash that button. Then place your note inside the brackets that appear.
After you insert it, hit "preview," and check your results. Whatever you put in the brackets will be invisible, on the public version of the page. If it's visible, try again.
BEFORE you try that, hit the "edit" button for this message. The 1st paragraph, above, contains two examples of the brackets, but ... THEY'RE INVISIBLE! You can only see the examples from the "edit" screen.