Translation:An apple is food.
I could've sworn it was or. But in this case, I wonder why "or" changed into "is".. (｡ŏ_ŏ)
What you are referring to is 또는 (or).
는 / 은 are topic markers. They signify what is being talked about. In this case, we are talking about the apple (사과) being food (음식입니다)
See the duolingo notes: https://www.duolingo.com/skill/ko/basics-1/tips-and-notes