This field allows you to enter in up to 200 characters about an item, which you may then have pop up any time that item is placed on an invoice, whether traded, sold, or returned. Whether it's to not forget an important accessory or to offer engraving services to the customer, or whatever else you may need, this feature can help greatly reduce employee error when handling items.
To turn on the popup notes, go to tools -> options -> inventory from the invoice screen, and check the option labeled "show item notes"