The alert provides the number of errors Googlebot encountered crawling your site, the overall crawl error connection rate for your site, a link to the appropriate section of Webmaster Tools to examine the data more closely, and suggestions as to how to fix the problem.
What you have to do when you get this message?
- Check the Crawl Errors page in Webmaster Tools.
- Check that pages on your site don't link to non-existent pages. (If another site lists a broken link to your site, Google may list that URL but you may not be able to fix the error.)