Kombu is dried edible sea kelp that is mainly used to make Dashi broth in Japanese cooking. It has a subtle but great savory taste (Umami) when it’s boiled in water. Kombu should be rehydrated in cool water and then cooked to extract its Umami flavor. When rehydrated, it grows about 3 times in size.
When Kombu is hydrated, it becomes a soft and tasty ingredient for Nimono (boiled vegetables and meat) and Tsukudani (strongly flavored Kombu).
You could substitute Katsuobushi, dried bonito flakes, in place of Kombu, if you prefer, although they don’t have exactly the same flavor.