don't know where you live and how well stocked your Asian grocery stores are, if you have one specializing in Japanese food, use that one (here surprisingly we can even get both konbu and bonito shavings in shopping malls that have a foodie kind of food department)! Dashi is the basic cooking stock for Japanese cooking. Soups, simmered foods, dipping and salad sauces, etc. The better the dashi the better the food! While there is instant dashi etc, if at all possible I'd try to make it yourself.
you'll need Konbu and kezurikatsuo.
Konbu is some sort of sea kelp (not to be confused with nori or wakame). it should look like this
: sometimes it is sold in wider, sometimes thinner strips, but it shold preferably be somewhat thick and dark and have some whiteish particles on the surface. often sold in packages like this
kezurikatsuo are shavings from dried katsuobushi, dried and fermented tuna (bonito) filets. katsuobushi blocks and the wood plane kind of thing needed to shave it can only found in japan, so go for shavings that are sold in packages, like eg. this
. if you have the choice, don't choose too fine shavings (hanakatsuo, which are used as a spice).
ichiban (primary) dashi:
1 liter cold water
30g bonito shavings
lightly clean the konbu with a damp cloth (you don't want to remove the white stuff) and put the konbu in water and heat slowly, so that in about 10 minutes it is just before boiling point. right before it boils, remove the konbu to a plate. (if you let it boil, it becomes slightly bitter.) bring konbu-water to a full boil, bring temperature down with a small glass of cold water, add the shavings, and bring again to a boil. as soon as it boils, remove from heat immediately and let the flakes settle down to the bottom of the pan. remove the foam at the surface, strain through a sieve with a cheesecloth. now you have ichiban dashi, which is very subtle and delicious and a great base for clear soups, sauces etc (and also miso soups).
niban (secondary) dashi:
leftovers from ichiban dashi
1.5l cold water
10-15g bonito shavings
add the leftover konbu and the leftover shavings from the primary dashi and the water into a pan and bring to boiling point, when reduce the heat and let simmer for 15-20min, or until the liquid is reduced by 1/3 - 1/2, depending on how strong you want it to be. add the fresh shavings and remove from heat. let settle, strain, and discard konbu and shavings. now you got delicious niban dashi, for simmering, thick soups, miso soups and what not.
sorry for the slight wall of text, but it's really quick and easy to do if you can get the ingredients! and opens the door to all kinds of amazing Japanese food.
(btw scales easily, so you can do 500ml water, 15g konbu & kezurikatsuo for only 1-2 persons, or double for more, etc)