Dashi is a stock flavored with dried fish flakes (typically bonito) and a type of sea kelp called konbu. You can make your own dashi stock, but it's far easier to use instant dashi powder, which is easily reconstituted in hot water. Use dashi stock as the base for Japanese dishes such as miso soup and hot-pots.

Related recipes:

Miso soup with tofu and mushrooms
Soba noodle scallops
Japanese-style grilled marinated tuna with wok-fried soba noodles