The Fail Message tab in Model Review controls the message displayed in the Results Browser once a check has been run that fails the given criteria.
At the top is a standard WYSIWYG HTML editor. Below in the body is where you would enter text, images, hyperlinks, tables and details to help aid the users in identifying why the given check may have failed. Areas with the % symbol are "dynamic text". When used they will be replaced with the actual check values in the Results Browser. The dynamic text can be added by right clicking in the body and selecting from the Insert flyout.