Saint-John's-bread vs foodstuff

Saint-John's-bread and foodstuff both are nouns.

Word	Noun	Adjective	Verb	Adverb
Saint-John's-bread	Yes	No	No	No
foodstuff	Yes	No	No	No

As nouns, foodstuff is a hypernym of Saint-John's-bread; that is, foodstuff is a word with a broader meaning than Saint-John's-bread:

Saint-John's-bread: powder from the ground seeds and pods of the carob tree; used as a chocolate substitute
foodstuff: a substance that can be used or prepared for use as food

Other hypernyms of Saint-John's-bread include food product.

Saint-John's-bread (noun)	foodstuff (noun)
powder from the ground seeds and pods of the carob tree; used as a chocolate substitute	(usually plural) consumer goods sold by a grocer
	a substance that can be used or prepared for use as food