The 2 characters "</" have been detected in a wrong place.
In most case, this is due to wrong javascript:
BAD <document.write("</h1>"); GOOD <document.write("<\/h1>");
W3C faq: http://validator.w3.org/docs/help.html#faq-javascript
HtmlHelp: http://www.htmlhelp.com/tools/validator/problems.html#script