Also known as "bonito flakes".
Cooked, dried, salted, smoked and fermented tuna.
The fish is smoked for an extraordinarily long time, sometimes for as long as a month. This completely dries it out; the final product is extremely hard, and resembles a piece of wood.
It's then shaved into very thin flakes to be used in cooking.
It's one of the two key ingredients in dashi broth, along with konbu. Konbu provides basal umami, in the form of glutamates, and katsuobushi provides synergistic umami, in the form of inosinate.