I’ve always wanted to know what the User Interface is, could someone explain it please? Thank you 
LAYERS
There are three layers in the game world where objects can live. The layers can be accessed by clicking the Layers button in the bottom toolbar to open the layers panel. Each game layer has its own object library that contains only the objects created in that layer.
By default, the Game World layer will be selected when the editor starts. The game world layer is the foreground of the game, where the action and game mechanics occur.
Behind the game world is the Background layer. Background objects have no physics (e.g., gravity or collisions), and do not contain behaviors or logic. When editing the background layer, the other layers will become translucent to provide a better view of the background.
In front of the game world is the User Interface layer. Objects in the user interface layer also have no physics, but they can have behaviors.
Tip: The user interface layer is a good place to put buttons, menus, or health bars, since they will render on top of the game and will not scroll.