https://kindle.amazon.com/most_popular

Not really sure what this means. Often, when I get a new book on my Kindle, there are already passages in the book that are highlighted, so I wonder if the "pre-highlighted" passages are part of these results.

It's explained at the top (through a link):

The Amazon Kindle and the Kindle Apps provide a very simple mechanism for adding highlights. Every month, Kindle customers highlight millions of book passages that are meaningful to them.

We combine the highlights of all Kindle customers and identify the passages with the most highlights. The resulting Popular Highlights help readers to focus on passages that are meaningful to the greatest number of people. We show only passages where the highlights of at least three distinct customers overlap, and we do not show which customers made those highlights.

The Most Highlighted of All Time passages have been highlighted by the most distinct customers. The Most Highlighted of All Time books are ranked based on a combination of number of highlighting customers and number of highlights. For Heavily Highlighted Recently passages and books the ranking is done in a similar fashion, but with a much greater emphasis on highlighting that was done recently over highlighting that was done a longer time ago.

It is completely meaningless. It is only a straw poll of what is being read at the moment, and has no more significance than that. Does anyone really believe The Hunger Games - good though the books were - are so inspirational?