Conceptually, scenes are the visual layer of your game: images, text, buttons, sliders, and anything else drawn on screen.
Technically A scene is a tree of nodes, edited visually on a canvas sized to your project's Reference Resolution. Every node's position and size is authored in that coordinate space.
Effectively a scene is just a view of your game. The splash screen is a scene, the main menu is a scene, and the game view itself is a scene.
See the Scene Editor for information on how the scene can be edited.
When a game is running the Scene Manager holds all the scenes in the game and manages loading and unloading scenes.
Sub Scenes are scenes that are designed to be loaded as sub element in a scene. These can be UI overlays (for reuse in multiple scenes), or a character made up of many small elements.
Scenes can also contain their own script that can either be embedded into the scene or in a Shared Script file that multiple scenes can use.