It’s an Instant Preview Troubleshooter. It send a request to the specified page of your site to create a small screenshot of it and crawls the page to create a preview that will appear in the SERP. You may need to request pages several times so all images can be cached and presented in the instant preview.

In conjunction with the Fetch as Googlebot tool you are able to see your site the way Google sees it in real time rather than waiting for it to be crawled, indexed and updated in the SERP.

One thing to bear in mind when using this tool is that each request you make using this will be considered a page view in Google Analytics.

So don’t be too surprised when you request every page on your site and end up with a huge spike in traffic!