Katsuobushi is a dried, fermented, and smoked skipjack tuna used in Japanese cuisine to add a deep umami flavor to dishes. It's often shaved into thin flakes and used as a key ingredient in making dashi, a traditional Japanese stock used as a base for many soups, sauces, and other dishes. The flakes are also commonly used as a topping for various dishes, adding a smoky and savory taste.