Anchovy stock is the Korean counterpart to Japanese dashi. Instead of bonito flakes, anchovies are used as the base for a fish stock that is every bit as savory and packed with umami, only even fishier in flavor given the nature of anchovies. If you compare the taste of bonito flakes versus anchovies, one is not better than the other; it's just a matter of what you're cooking and what flavor profile you prefer. Just as dashi complements miso paste, anchovy broth is an ideal counterpart to Korean gojuchang paste (a chili paste with soybeans and other ingredients).
You'll find dried anchovies at any Korean supermarket, along with dried pollack, dried squid, and other dried seafood used for stocks. Anchovies will come in several sizes, the smallest no larger than one or two inches in the length. You can buy a few varieties if you're curious about subtle differences, but anchovy stock is anchovy stock, no matter the size of the fish.
There are variations on how to make the stock. The simplest method is to boil anchovies in water for ten minutes, then drain the liquid and discard the anchovies. You'll get a pure and clean-tasting broth that provides plenty of fishy flavor, though not a lot of depth. Another common stock includes kelp, the same kind used in making dashi. You simmer the kelp along with the anchovies for ten minutes; drain, then discard the anchovies. For even more depth, some recipes for anchovy include cubes of daikon in addition the fish and seaweed, which is is a great option if you happen to have daikon and want to eat the cubes afterward.
Anchovy stock is a common stock used in beloved Korean dishes such as soondubu or kimchi stew, and it is a built-in step in all kinds of seafood stews. Like any seafood stew that starts with fish bone stock, clam juice, or some other form of pure-tasting broth, the anchovy stock acts as the base for the other types of seafood you add to the pot. By the end, you'll have a seafood broth that's light in body but fairly complex in taste, and the note of chili paste or powder acts as a counterpart to the intensity of the seafood flavor. Vegetables such as daikon and greens like watercress are common additions, but don't feel limited by those alone. Best of all, once you've gotten a feel for how these seafood soups behave, you can vary the kinds you add—fish, shrimp, squid, clams, mussels, and so forth are all delicious options.