A version is a copy of the whole deck: text, order, hidden slides and design. Restoring one can be undone.
With this on, the file shows a "Save as PDF" button and prints one slide per page. With it off there is no button and printing the page gives nothing. Someone determined can still get at the content, as with any web page.
The doc must be shared as "Anyone with the link can view". You see what will change before anything does, and the doc itself is never changed. Importing again updates the deck: your edits, order, design and images are kept. Sections whose heading is gone from the doc are kept too, unless you tick the box above; sections you made in the app always stay.