He's a long time employee of Google who now runs their spam team. He frequently posts videos and blog posts assisting webmasters in avoiding spam penalties with Google, and encouraging them to use best practices on their sites according to Google's policies. Every time he posts something, all the SEO blogs explode with posts and comments chiming in with their opinions. It's almost obnoxiously obligatory to voice your thoughts any time he says something, otherwise risk looking irrelevant.