A free trade zone enables a manufacturer of, say, laptop computers, to import parts from another country, keep those parts within the free trade zone so they are technically "not entering the USA" and tariffs etc. don't have to be paid, manufacture the product using those parts, and only deal with customs/taxes etc. when the finished manufactured product leaves the FTZ.
Anyway that's my understanding.
You will find these FTZs in the strangest places … like Research Triangle Park, NC.