If you are a collaborator on a base and you have creator, editor, or commenter permissions, you can comment on individual records in a table.
Comments are shown in the record's activity feed after the record has been expanded. Once the record is expanded, you will see the record comments in the activity feed, along with the record-level revision history. (Note that although your revision history is limited by your workspace's payment plan, comments will remain forever until manually deleted.)
You can see at a glance which records have comments on them by seeing the clickable comment icon in the record's primary field, with the number of associated comments inside.
When in an expanded record, you can type a comment in the text box at the bottom of the activity feed.
You can @mention a collaborator in the comment box by typing the @ symbol or clicking the @ icon in the bottom right of a comment box. This will give you a dialog that lets you select a collaborator.
When you @mention a collaborator, they will be notified that you mentioned them in the given record. As you type your message in the comment box, it will show which collaborators will be notified when you post your comment. To post your comment, press the Enter key.
Note that if you @mention a collaborator in a comment, that collaborator will automatically be set to watch that record—meaning that they will be notified about further comments you make on that record, even if you don't directly @mention them.
You can also @mention a collaborator in a long text field. (This will also send the @mentioned collaborator a notification.)
To delete or edit a comment that you previously made, simply click on the menu button to the right of the comment to bring up those options.
(If you wish to create a clean copy of the base without comments, you can duplicate the base and uncheck the Duplicate comments box.)
Two short video tutorials below can give you more information on commenting and @mentioning collaborators: