The Program output of your SlingStudio hub is what it's currently outputting, either to a storage device, to live video, or both. Program output can be live switched and live edited.
Preview is an internal-only view of your video that you can use privately to sketch different compositions, text overlays, lower thirds options, and more. Preview shows you the scene that will go on air (program) when the next transition occurs.
Manual transitions can be made either by dragging and dropping the video image from Preview to Program, or by tapping/clicking the Preview -> Program button.