You have only half the doctype - missing the full URI including the location of the DTD. For more information this
http://www.alistapart.com/articles/doctype/ webpage is very useful.
Validation is basically making sure the markup/code used in your documents adheres to the guidelines set down by the W3C (World Wide Web Consortium - the 'governing body' of the web). Have a read of this
http://www.motive.co.nz/glossary/validation.php?ref
as they put it better than me.