Being able to leave comments in a workbook is a saving grace when you’re confused about a formula or a data series and you can’t ask the person who might be able to help you in real time. You can instead leave a comment in the workbook to which that person can respond.
Click the cell to which you want to add a comment.
Click the Review tab.
Click New Comment.
Type the comment that you want to add.
Click anywhere outside the comment to discontinue adding text.
To view a comment on a non-touchscreen ...