While not especially popular globally, they’re exceptionally valued in Chinese cuisine. Maw are considered such a delicacy and are so valuable in China that people have been arrested for smuggling it in on the fish maw black market.
While all fish have the organ, for Chinese culinary purposes the most commonly-used fish maw come from larger fish. They’re dried in commercial preparation and then must be rehydrated for many hours — often overnight — to be cooked. Mostly flavorless, fish maw is primarily used for its texture and flavor absorption.
They’re most well-known usage is in soups; often simmered or braised for a long time, fish maw take on the spices and other flavors used in the soup and add texture. They’re also high in collagen, so have that health benefit while also adding the benefits of collagen to the soup itself.