Click the icon in the upper right of an element to trigger the Comment popover.
You can add a new comment, reply to an existing thread, or edit or delete any comment you authored. You can mention other users in your organization with @ tagging, and they will receive a notification via email.
By clicking "Resolve", you effectively "archive" a comment to the Resolved tab.
You can unresolve a comment to move it back to the main view.