A Scene represents a location in space from which a panoramic image is created. That location could be a real world location, captured with a 360 panoramic camera, or a synthetic location created with any number of 3D modeling or CAD tools.
A Scene has associated with it a 360 panoramic image, ideally in stereo (separate images for left and right eyes), and a description that helps people understand the context for the Scene.
A Project can have up to six Scenes associated with it.
Just in case you need a reminder of how to access these different content types, you can refer to the quick reference image at Web Portal: Scenes And Hotspots.