A 'change comment' is a short description that details the changes made to a page during an edit. Change comments are a useful way of keeping track of the history of a page.
A change comment is not the same as a comment added to a page. Refer to Commenting on a Page for information about that type of comment.
Note that once a change comment has been added and the page has been saved, it is not possible to update or remove the change comment.