What is Dashi? It forms the base for miso soup, clear broth, noodle broth, and many kinds of simmering liquid.
Before starting to explain about Dash, allow me to mention that in Japan, water is soft. There are two kinds of water - hard and soft, and they are defined due to contents of calcium ion and magnesium ion in the water.
Soft water is perfectly suited for Japanese cuisine, thus it has developed Dashi culture. Hard water contains more magnesium and calcium, which makes it more difficult to dissolve particles.
The most common form of dashi is a simple broth or fish stock made by heating water containing kombu (edible kelp) and kezurikatsuo (shavings of katsuobushi - preserved, fermented skipjack tuna) to near-boiling, then straining the resultant liquid. The element of umami, considered one of the five basic tastes in Japan, is introduced into dashi from the use of katsuobushi. Katsuobushi is especially high in sodium inosinate, which is identified as one source of umami (savory taste.)
Kombu (kelp) dashi is made by soaking kelp in water.
Niboshi dashi is made by soaking small dried sardines (but remove the heads to prevent bitterness) in water.
Shiitake dashi is made by soaking dried Shiitake mushroom in water.
There are many kinds of instant Dashi powder available. It can be powder, in sachet, or concentrated liquid form.