Biff says" i realized what a ridiculous lie my whole life has been." What does he mean ?

This is Biff coming to terms with the fact that his father's illusions of success for him were truly just illusions and nothing more. Biff has spent his life trying to live up to - or react against - an impossible falsehood and a vision of himself that never existed. Willy's illusions about success impacted every part of his sons' lives.