(from "With Other Natural Flavors
In the food industry, any "natural" flavoring additive which enhances the taste of other natural flavors.
Berries, for example, taste great when they're fresh, but their taste is difficult to recognize in processed foods. (More precisely, you can use strong enough concentrations, but it ain't cheap, and cheap is the name of the game.) The manufacturer of (let's say) a certain fruit leather that can be rolled up can add WONFs to fool your tongue into thinking you're tasting strawberries when you're mostly tasting inexpensive, vat-processed chemicals.
Because WONFs are, in a technical sense, "natural", the manufacturer can still legally claim (in the US, anyway) that their product is all-natural.