W3C Compliant Issue!

Hey guys, So I'm having an issue with the site I'm working on now. I've been using http://nibbler.silktide.com/ to see the performance of my site and one of the main issues came up as ALOT of pages not being W3C compliant. How can I fix this??? I'm Using wordpress with a template by the way.

W3C compliance means that something in your HTML, CSS, JS, HTTP, etc isn't validating. Your site could still function properly even though it doesn't validate. Your character encoding could be set wrong. You could be using HTML5 tags with an XHTML doc type. You might be using quotes in content instead of using "

You would have to get a lot more specific for us to help you.

I remember when I first time saw this (I was a complete newbie) and I got scared.

Now I couldn't care less about W3C compliance.