It is not a tax on imported goods only, it is a tax that is collected by the government (not here yet) every time something gains value through a production process. So as a raw material is processed into something more valuable, like iron ore into steel, then steel into sheet metal, then sheet metal into a particular part, etc. The iron would be taxed every time it gains value until it is sold in its final retail form. Look it up on wikipedia, it is too complicated to explain here.