Before we go further, it’s better to know what exactly W3C is.
W3C (World Wide Web Consortium) is an international organization, with more than 400 members, and their main job is developing web standards. W3C also engages in education and outreach, develops software and serves as an open forum for discussion about the Web.
W3C validity, mainly covering the validity of syntax in the markup language (XHMTL, HMTL, etc), CSS, and (sometimes) RSS/Atom Feeds. With the W3C guidelines, all syntax will be more structured and read perfectly by all browsers.