We are using a concept called Parenting. When you create a group of transforms, the topmost transform or Scene is called the “parent transform”, and all transforms grouped underneath it are called “child transforms” or “children”. You can also create nested parent-child transforms (called “descendants” of the top-level parent transform).
Here is an example scene hierarchy with multiple transforms with parent-child transforms.
In this example
Click a parent transform’s circled "-" / "+" icon (on the left-hand side of its name) to show or hide its children.
To make any transform the “child” of another, drag and drop the intended child transform onto the intended parent transform in the Scene Tab.
To make any transform in the root of a scene (not child of any transform), drag and drop the intended transform onto the blank area of the Scene Tab.