You can add comments to folders and individual files to give other users information or notes.
You can comment on both files and folders. This feature is available in a file preview screen, in the Site Content dashlet, and in several of the Document Library views.
Note: You can’t add comments if your permission level on the site
is set to Consumer. Speak to your administrator if you need to change your
permission level.
Your comment appears at the top of the list. You have the option to edit comments you’ve made, and site managers can edit any comments.