The word back has several meanings:

1. The rear part of something, opposite the front. The back of a book is where the story ends, the back of a classroom is farthest from the teacher's desk, and the back of your body is the side you can't see without a mirror. When you turn your back on someone, you're literally facing away from them.
2. To return to a previous place or position. When you go back home after school, you're returning to where you started. If you read a paragraph again, you're going back over it. Someone who comes back from vacation has returned from their trip.
3. To support someone or something. When you back your friend's idea for a class project, you're saying you think it's good and you'll help make it happen. A person who backs you up confirms your story or stands with you when others disagree. Politicians look for supporters to back their campaigns.
4. The part of your body between your neck and your tailbone. Your spine runs down your back, and it's what lets you stand upright. A backpack got its name because you carry it on your back.

The word appears in many expressions: back and forth means moving in both directions repeatedly, behind your back means without your knowledge, and back down means to stop insisting on something or to admit you were wrong.