Orders in the admin have an option to add comments, these are displayed towards the bottom of the order page.
Order comments should be used to provide further information – to yourself and anyone else using the admin – regarding the order, customer or anything else that may be relevant.
Order comments are not shown to the customer by default and are for internal use only.
Comments can be shown to the customer by ticking the “This comment can be viewed by the customer” box, this will display the comment to the customer as part of their order when in the storefront account area.