Parenting game objects
Each game object can have a parent. This means that the game objects will move, rotate, and scale along with its parent.
I could use many words to explain this concept, but there are things which videos can explain better than words. As such, there is a very short video explaining, The Hierarchy and Parent-Child relationships at: https://unity3d.com/learn/tutorials/topics/interface-essentials/hierarchy-and-parent-child-relationships.
I recommend that you to watch it before you continue reading this book. I'll wait for you here.