It means that there is no duty charge which is a tax. The bottom line is that duty free items are generally less expensive. Liquor for example is much less. A bottle of Absolute will be $5.95 to $10 v. around $20 in the Sates.
Items like liquor, cigarettes, perfume, jewellry, and cosmetics are heavily taxed (luxury tax) because they are non-essental items. For example, a bottle of Kahlua here is about $22.00. I get it in Mexico for about $10.00 a bottle. You don't pay U.S. taxes because you are not purchasing the item in the U.S. (which is also why the duty-free shops onboard don't open until the ship is in International waters).
It means items may be purchased in these locations and they will qualify toward you tax free liimits, those limits vary depending on your citizenship, where you are going, where you buy, and how long you are out of the country.
Note: duty free at the cruise port or at the airports in this country are only duty free to aliens purchasing here and returning to other countries, not duty free to Americans.