HS Codes are short for Harmonized Commodity Description & Coding System. These codes are used to describe the class of product that you are shipping internationally. It is imperative that you enter the correct HS Code for each product you're shipping in the commercial invoice. If you are not sure, call your customs broker and ask them which HS code you should be using for the product being shipped.
If you do not have a broker, you can call the courier fulfilling the shipment on ShipTime and ask them. Information on HS Codes can also be located on Industry Canada.