I think, and correct me if I'm wrong, that he means to wash your mind clean from your past bad experiences at previous 121 employers and start new. The instructors in the schoolhouse have no time or patience for that stuff. They will be the first to tell you to shut up and move on. It's pretty annoying to hear new guys who are jaded complain over and over again how terrible their old airline was and how things were over there. A positive attitude will get you a long way here.