No, it does not create a copy. It creates a new scene that inherits from another scene. Inheritance is a concept from object oriented programming. The new scene will contain all the same nodes as the original, but you can then add additional functionality. If you ever change the original scene, the inherited one will also update.
For examples of how this is used, see this tutorial: http://kidscancode.org/blog/2018/01/godot3_inheritance/