Source can't be found
Your experiencing this error because a page has been moved or some silly person has created a broken link!
What are broken links?
Broken links fail to sent traffic to a source that may or may not exsist, the issue is that the link is incorectly set so the server gives you a warning.
What is an error 404?
An error 404 and specifically in this instance is a message from the server telling the web user that the information is unavalible.