in the back
1. In an area physically behind or at the back of something else. We keep those items in the back of the store—here, let me show you.
2. In a particular area that only some people, often employees, have access to. That coffeemaker is completely out of stock—we don't even have any in the back. Jason's in the back—do you want me to get him for you?
in the back
in the back part of a building; in the back room of a building. I don't have your size here, but perhaps I can find it in the back. He's not in the shop right now. I'll see if he's in the back.