Eel sauce (kabayaki sauce) is a sweet grilling sauce made with mirin (or sake), sugar and soy sauce. It's frequently referred to as eel sauce because the mixture is used to prepare unagi (fresh water eel). The sauce can also be used for grilling meats and other fish. You can purchase commercially prepared sauce or make your own.
You can purchase this sauce in Japanese markets or online specialty stores. In Japanese markets it may be labeled as Kabayaki, Unagi Sauce, or Sushi Eel Sauce. Amazon carries several brands: Eel Sauce.
If you don't have eel sauce, you can use one of these very good substitutes:
Recipe To make about one scant cup of Eel (Kabayaki) sauce:
Combine 1/2 cup mirin or dry sherry, 1/2 cup soy sauce and 1/4 cup sugar. Place the mixture in a small pot and bring it to a boil. Turn the heat down and reduce the mixture by about 1/3. Allow the sauce to cool then transfer it to a jar with a tight-fitting lid. You can double or triple the recipe if this is something you use a lot.