A lot of businesses like stores, restaurants, and airports have public areas that customers are allowed into, and other rooms that only the staff or managers can enter. The private rooms are sometimes called "back rooms".
We associate the "back room" with dark, sneaky, secretive actions. For example, in politics, people sometimes talk about "back-room dealings". These are agreements that politicians make secretly, without telling the public. "Back room dealings" are bad and maybe even illegal.