HS code is short for Harmonized Commodity Description and Coding System. It's a list of numbers used by customs to classify a product and it is used to define the duties and taxes to be paid.
First, select the country/territory the order is shipping to. Then, input the product you are looking up in the "Search For" section (i.e. for algae, type 'algae', for medium, type 'media', etc.)