Introduction
User interface refers to the methods and components through which a user will interact with your application, which may or may not be a game. To make your life easier, Libgdx provides you with a built-in variety of widgets and structural components you can use to create a scene graph, commonly known as Scene2D.
Note
A scene graph is basically a data structure to hierarchically organize the content of a scene. This implies that changes in parent nodes will be reflected on their children too.