"An apple is food."
Instead of a subject particle (가/이) they used a topic particle (는/은). In this sentence there is really no difference in meaning between the switch, but as you continue, you'll see that the topic particle is for, naturally, the main topic of the sentence, and the subject particle is, naturally for the subject of the topic. Easy example: 내일은 제가 공연에 가요 = Tomorrow, I'm going to a concert.
You can also use topic particles to differentiate two different things or people. 저는 딸기를 좋아해요. 그런데 누나는 딸기를 좋아하지 않아요 = I like strawberries but my older sister doesn't.