Airtable gives you the ability to 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 team's payment plan, comments will remain forever until manually deleted.)
You can enter 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.
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 made, simply click on the Delete and Edit buttons below your comment.
(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: