Dashi is a very flavorful basic broth in Japanese cooking. It has a savory taste (umami) and is typically made from dried bonito flakes (Katsuobushi), dried fish (Niboshi) or kelp (Kombu). It is a very important ingredient for traditional Japanese food. Dashi is used not only for soups such as Miso Soup, but also for a lot of boiled and flavored vegetables and stews.
Katsuobushi, Niboshi, and Kombu are all easily found at Japanese markets or online stores.