1. Build website 2. Go to link http://validator.w3.org/ 3. Paste website address into address bar 4. Click check 5. Look for the errors 6. Spend countless hours fixing every single one of them.
It's a validator that checks the coding of your website to see if there any errors... go to http://validator.w3.org Just type your web address and check for errors, if there is any correct it. But you should have knowledge of coding to fix it.
You do not need to stress over every little error from W3C validation. Even the Google homepage does not validate properly.
@w3bmaster - it doesn't directly impact SEO but validating your html and css code means that its standards compliant and therefore easy for the search enginr roblts to parse