Duty Free goods were sold to the vendor without import or export duty added to the price.
In theory, he can then sell those goods to you without adding duty to the price you pay to him.
That's what normally happens.
This can be different from Tax Free Goods.
There are some locations where Duty Free goods are sold, but sales tax must be added to the final price. There are other places where Tax Free goods are sold, but duty is added to the price. This is all governed by local import and export laws.
Buying Duty Free - or Tax Free - goods does not mean you will not have to pay Duty or Taxes at some point or place.
It only means that the seller is selling them to you without adding any Duty or Tax at the point of sale.
When/if you try to take those Duty Free / Tax Free goods into another country, you may be required to pay duty or taxes according to the import laws and regulations of that particular country.
Buying Duty Free or Tax Free goods does not guarantee a good price for you. It only means that duty or tax are not added. It doesn't mean that the seller cannot add a 200% profit margin onto the price hs is asking.
Most Caribbean "Duty Free" shops are an example of this.
The shop owners know that naive cruisers are looking to buy things "Duty Free" in the islands to get a bargain. So they sell duty free, but the total price is higher than what you might pay at home.