With Google Instant, search results show up instantly as you type your query. This lets you fine-tune your search on-the-fly to find exactly what you need. Of course, you're limited to narrowing things down based on a page title and a brief description. How many times have you found a page with the exact phrases and keywords you needed in the title, but nothing useful once you clicked through? Google Instant Previews helps you save that unnecessary click by giving you a glimpse into the content from the results page:
Instant Previews provides a graphic overview of a search result and highlights the most relevant sections, making finding the right page as quick and easy as flipping through a magazine. To use it, click once on the magnifying glass next to the title of any search result and a visual overview of the page will appear on the right. From there, hover your cursor over any other result to see a preview.
According to Google, "people who use Instant Previews are about 5% more likely to be satisfied with the results they click." The feature is now rolling out and should be available in more than 40 languages in the next few days.
Previews don't count as clicks or affect your analytics in any way. This new feature doesn't change how Google ranks pages or change their algorithms. Still, it will probably change user behavior and may affect your numbers. Check out the Google Webmaster blog for details there.