O'Brien makes this statement in 1984 as he explains to Winston what the new world the Party is creating will be like. O'Brien first asks Winston how one person asserts power over another. Winston, who has been long subjected to life in Oceania, and now to torture, answers that it is by making the weaker person suffer.
O'Brien agrees, explaining that you can't know for certain a person is obeying your will and not his own unless he is suffering. He goes on to paint a picture of a new social and political order in which nothing will matter but power. This does not hint at dystopia: it is dystopia. Facts and science won't matter: they will be whatever those in power say they are. Further, as the single-minded focus on power grows, people will become ever more merciless. Human relationships will be destroyed--O'Brien points out that this is happening already--and people will be taught to experience only four emotions: "fear, rage, triumph, and self-abasement." There will be no art or literature. There will be no distinction between beauty and ugliness. People will live miserably. However, O'Brien says:
But always—do not forget this, Winston—always there will be the intoxication of power, constantly increasing and constantly growing subtler. Always, at every moment, there will be the thrill of victory, the sensation of trampling on an enemy who is helpless.
He then says:
If you want a picture of the future, imagine a boot stamping on a human face—for ever.
In other words, O'Brien is giving Winston (and the reader) an easy-to-remember image to capture what he is saying. Humanity, represented by the human face, will be stamped out by an increasing exercise of violence and power, represented by a boot. Orwell was referencing what already had happened in the recently defeated Nazi Germany and what was continuing to occur in the Stalinist U.S.S.R. He was warning that this could spread if people weren't vigilant about holding onto relationships, language, and political power.