You can add a comment to a clip for other users to see by clicking thebutton. Enter your comment into the pop-up window and click "OK" when you're done to save your comment.
The comment will display on the clip right above the play bar as shown below highlighted by the red oval. An orange comment icon will also appear on the play bar once a comment has been created as highlighted by the red circle. Keep in mind that the comment appears at the same location as the playhead when you click the Comment button.