The main function of an adjective is precisely to modify a noun, often by giving a characteristic to it. There are many different adjective phrases,which are sentences in which one adjective modifies a noun, sometimes using intensifiers and other features. In this case, the adjective in its superlative form is the best and the word it is modifying is salsa, because according to this sentence, the tomatoes from the garden have the particular characteristic of making nog simply good salsa, but the best one, the one with probably the best taste, color, or any other feature.