A tariff code is a unique code that's assigned to every type of product, used by customs authorities around the world to determine the level of import duty and tax to be applied.
Sometimes known as a harmonisation code, tariff codes are maintained by the WCO. Every tariff code has at least 6 digits, and no more than 10. The more digits the code has, the more specific it is. For example, 'Shoes' may have 6 digits, but 'Leather shoes' may have 8 digits.
Most customs authorities provide a web portal where you can check the correct tariff code for your product along with the applicable rate of duty and tax that will be applied when importing it into that country. Here's a link to the UK tariff codes.