In Card windows, there is a Pencil icon at the top of the window. This is the Comment button. Click on it will open the comment window. You can enter comment related to the record in the card window.
Please note that the icon will change to a pencil with several lines if comment have been entered. This will help you to identify which record has comment and which record does not has comment at the card form itself, otherwise, you need to click on the pencil icon one by one to check on the comment.