Wikipedia provides a pretty comprehensive explanation and unravels the mystery for Dashi.

Dashi is a class of soup and cooking stocks considered fundamental to Japanese cooking. Shizuo Tsuji wrote in 1980 that "many substitutes for dashi are possible, but without dashi, dishes are merely a la japonaise and lack the authentic flavor."

Dashi forms the base for miso soups, clear broth soups, Japanese noodle broths, and many Japanese simmering liquids. The most common form of dashi is a simple broth or stock made by boiling kombu (edible kelp) and kezurikatsuo (shavings of katsuobushi) and then, straining the resultant liquid.

Fresh dashi made from dried kelp and katsuobushi is rare today, even in Japan. Most people now use granulated or liquid, instant substitutes. Other kinds of dashi stock are made by soaking kelp, niboshi, or shiitake in water for many hours or by heating them in water nearly to boiling and straining the resultant broth. 

Kombu dashi stock is made by soaking kelp, or sea tangle, in water. Niboshi dashi stock is made by pinching off the heads and entrails of small dried sardines, to prevent bitterness, and soaking the remains in water. Shiitake dashi stock is made by soaking dried shiitake mushrooms in water.