Just means your XML sitemaps have to error free. Google Webmaster Tools will tell you if anything is wrong. You might check this article about XML sitemap validation. But really, any sitemap generator will generate correct XML sitemap files. And even if something is wrong, Google Webmaster Tools will tell you.
you can validate your sitemap through this URL. http://www.xml-sitemaps.com/validate-xml-sitemap.html Code (markup):
Xml sitemap validation means submitting xml sitemap in Google Webmaster Tools in order to check whether the XML sitemap created is correct or not.Google Webmaster wont index those pages inside xml sitemap which contain some error and also if there is any error while generating xml sitemap in xml sitemap,such xml sitemap wont index in Search Engines..!
Sitemap validation is to validate your Google sitemaps and f you have error you will see the line number where it is.