Your back is your spine, or the rear part of your whole body. When you lie on the grass on your back, you can watch the clouds in the sky above you.
There are dozens of ways to use the word back. You might have back pain, or stroke your cat's back. Your clothing also has a back, and you have a figurative back: "Get off my back, Mom!" You could request a table at the back of a restaurant and then lean against the back of your chair. You can also "answer back," or reply, or choose to back a friend's fundraising project — give her money, in other words.