Duty drawback is a refund of duties, fees and taxes paid on goods imported into the U.S. that are subsequently exported from the U.S. Similar to how you are refunded sales tax when you return an item to a store, you can claim a duty refund when you export an item that was previously imported.
There are several different categories of duty drawback, but the two most common categories are manufacturing and unused merchandise.
Manufacturing duty drawback applies when you import an item that is then manufactured into a different item. For example, if you import bicycle tires and export finished bicycles, then you can get the duty you paid for the bicycle tires refunded when you export the finished bicycle.
Unused merchandise duty drawback applies when you import something, and then export it in the same, unused condition. If you import bicycle tires, and then export them again without changing them, then you can get a refund for the duty you paid when you export the tires.