your first source should be the documentation:
in the documentation is a tutorial section. in this section, scenes and nodes get explained:
my short explaination: a scene is build from many nodes. its like a container.
nodes are gui elements, 3D objectst, 2D Images -> stuff you can see
but nodes are also music player, timers etc -> the background stuff in your game
nodes can be childs of other nodes. this means if you move the parent, the childs will "follow".
nodes can have scripts attached to them to give them "behavior".
you can make a game consisting only one scene and showing/hiding the nodes you dont want (show/hide main menu).
i use scenes to structure my project: a scene for the start menu, a scene for the world map, and scenes for my towns for example.
you can re-use scenes in other scenes: i have a main menu scene that i include in my world and town maps.