Dashi is the most basic stock used in many japanese dishes. it's a stock made from konbu (dried kelp) and katsuo bushi (which is rock-hard dried bonito fish shaved into flakes, or bonito flakes). in the olden days (n i believe in the rural areas of japan now), people take a "cake" of dried compressed bonito fish and shave it when needed but now, packets of shaved bonito flakes can be purchased at supermarkets.
The traditional way of making dashi is quite troublesome n expensive. you need to buy dried kelp and bonito flakes. first wipe clean a piece of konbu (6 X 4cm). put it in a pot of 7 cups of water. just before the water comes to boil, remove kelp and discard. sprinkle in 50g of bonito flakes (which is a lot because they're light as a feather... they will fly all over the place one hor) and remove pot from heat. as soon as the flakes sink, strain stock and discard flakes. can be kept for 3 days in the fridge. i did this once for a steamboat party and never tried again LOL.
however, if u only need like 100ml or 50ml, then what r u going to do with the remaining dashi?? very wasteful right? so dashi-no-moto or granules are a convenient alternative. they come in a box of 5 or 10 sachets. each sachet can make 600ml of dashi. if you need only a little, you can scoop out some, fold the sachet to close it and keep the remaining powder in the fridge. it's kinda like our chicken cubes (which means it contains some MSG). if u can read chinese, the bottom left says 粉末 which means "powdered-form" and if you can read japanese, かつおだし (katsuo dashi) is what you're looking for. meidiya, isetan and cold storage have this. yamakawa may have, i'm not sure. if i go there, i'm usually more interested in the snacks LOL NTUC not sure too.
there are also dashi bags which work like tea bags and contain pure ground kelp and bonito. i used to use this until pochacco intro me to dahi-no-moto ^^ i find this healthier (cos no MSG) but again, there's a lot of wastage because one bag makes a couple of 100ml of dashi and u can't use 1/2 a bag if you need less... so i guess, dashi-no-moto is still the winner when u need just a little. u can even share one box with a friend (5 sachets will go a long way) to cut cost.
hope this helps =)