The Google Rich Snippets Testing Tool is used to check that Google can correctly parse your structured data markup and display it in search results.
Rich Snippets appear to be becoming more important for SEO especially with local search or review sites.
You can learn more about Rich Snippets and structured markup at http://schema.org/ who say,
“A shared markup vocabulary makes easier for webmasters to decide on a markup schema and get the maximum benefit for their efforts. So, in the spirit of sitemaps.org, Bing, Google and Yahoo! have come together to provide a shared collection of schemas that webmasters can use.”
If the information on scema.org is all a bit double dutch to you, feel free to get in touch as I will soon be putting together some simpler instructions on the basics like Person, Place, LocalBusiness