By cloning prefabs, you have added a house and some objects to an empty scene. Additionally, you have added a keyboard-controller character to the scene.
By childing the Main Camera to the character GameObject, the Main Camera maintains the same position and rotation that is relative to the character at all times. Therefore, as the character moves, so does the Main Camera, giving a simple, over-the-shoulder type of viewpoint for the game action.
Remember, because the Main Camera is now a child of the ThirdPersonController character, the values of the Transform component's Position and Rotation are relative to the Position and Rotation of its parent. For example, the Main Camera position of (0, 3, -4) means that it is 3 Unity units above the ThirdPersonController (Y = 3) and 4 Unity units behind (Z = -4).
