A Google Sitemap is a file which contains URLs and some additional information for all public pages or documents of your website. Google reads this file and adds the page to their index.
Please read entire post: http://www.blogworld.com/2011/01/13/30-days-to-a-better-blog-install-google-sitemap/ (They have instructions for wordpress and blogger)
Now I'm going to be honest I have no idea what it does but I'm sure its something very important. I need to read up more on this feature. TO DO!