海鮮 is seafood (海 is ocean and 鮮 is fresh, so seafood is literally ocean fresh). 醬 is sauce, so altogether 海鮮醬 is seafood sauce (even though it doesn't contain seafood, so if you see a bottle labeled vegan hoisin sauce, well, obviously).
About This Dish
Hoisin sauce is a dark, thick sauce made from soybeans and other flavors and spices.