This is an excellent sauce used as a marinade for grilled or baked fish. Try it as a base for fish and seafood soup or similar simple dish. You'll recognize the flavor instantly if you've ever eaten at a Japanese restaurant.
You'll find miso in the Japanese foods section of most grocery stores. It's a paste consistency which can be used as is to make spreads or can be diluted with other ingredients, such as in this recipe, to make marinades and soups.