What does "the terrible twos" mean?

Context:
Even so, for most people, this stage of development is blessedly temporary. By their mid-twenties they are moving beyond narcissism, usually because they have started a family and must provide for their children, which requires sacrificing their own pleasure and convenience. But in the interval between the onset of adolescent narcissism and its gradual dissipation in light of adult responsibilities, there is an extended period akin to the terrible twos, which can be summed up in one word: me. Or more precisely: me, me, me, me, me!

More:
http://michaelprescott.typepad.com/michael_prescotts_blog/current_affairs/