The Timeline appears when you switch to Animate mode. It displays playback controls and options for the current animation. When the Timeline is visible, the Hierarchy displays an Animations pane, which contains a list of all the animations that are available for the current artboard.
Learn more about the Timeline in the Animation section of this manual.
The stage is where your designs and animations are rendered. Rive's stage is infinite and it allows you to place an infinite amount of artboards on it. When you first create a file, a default artboard is created on the stage for you. Every object on the stage must be inside an artboard. That means that every Rive file has to have at least one artboard.