Translation:Bread is food.
Because of the 은 / 는 topic marker, the sentence usually has more context. My friends always said the topic marker often means ''as for..'' so here my initial answer would have been: As for the bread, it is food.
When there was a sentence asking to write "A book is a thing/Books are things" (책이 물건입니다, according to Duolingo...), it did not accept the particle "은". It seems to me that this sentence here (about the bread) is very similar grammatically to the other one. So... what is the explanation for using 은/는 and 이/가 in this context?
이 would make the sentence: "the bread (like, one specific bread that youre talking about) is food", whereas 은 makes the sentence "bread (in general) is food. Basically think of 이 as talking about something specific with and 은 as talking about something in general.