After weeks of torture, O'Brien tells Winston that he is about to enter the second stage of the three-stage process of "reintegration": learning, understanding, and acceptance.
O'Brien reveals a cornucopia of information to start the process of reintegration: the Party is indestructible because it seeks absolute power for power's sake; the Party will succeed indefinitely because it controls the only reality that matters — the human mind; the Party shall eventually be rid of all enemies because all private loyalties will be abolished.
O'Brien likens a picture of the future to be "a boot stamping on a human face — forever." It will be a hateful world of power, manifesting itself in "inflicting pain and humiliation."
O'Brien forces Winston to look in the mirror for a picture of "humanity." Winston cries upon seeing his deterioration; he looks to be 60-years-old with the grayness, emaciation, and a not-so-straight spine. In short, he looks like a boot stamped on his face.
O'Brien humiliates Winston by ridiculing this picture of "the last man." (An allusion to Orwell's originally intended title, "The Last Man in Europe.")
Winston lashes back, and O'Brien recognizes that there is one last strength: Winston has not yet betrayed Julia.
Whatever, says O'Brien. It doesn't matter since everyone gets shot anyway. Although, better play it safe and destroy that last loyalty.