This field is in the Published block, top right of your edit page.
It’s not required for small changes but if you make a major change to a page it’s useful for explaining to others in your team or future editors what you’ve changed – and why – on this page.
It’s uncertain at time of writing whether this message will ever surface on the published page. If this is confirmed, write a brief public change note such as ‘Replaced XX document download with revised version’