This tool checks for any content that may be classed as "cloaked" from search engines. This is text that you supply to Google but not to your visitors. Cloaked content is against the Google webmaster guidelines and can result in a severe penalty should it be discovered.
You can use this tool to discover if you have this on your site simply by entering your URL. It compares your site using a regular browser and the Google Bot browser user agents. If you have cloaked content, you should remove it immediately.