His subversion having been exposed, Winston is being brutally tortured by O'Brien. He's strapped down to a cot, hooked up to a machine that emits excruciatingly painful currents of electricity. As Winston's interrogator, O'Brien can turn up the machine's dial if he doesn't get the answer he wants.
O'Brien tells Winston that the truth is only what the Party says it is. He reminds Winston of something he once wrote in his diary: "Freedom is the freedom to say that two plus two equals four." O'Brien holds up four fingers and asks Winston how many fingers he's holding up. Winston answers four, prompting O'Brien to turn up the dial on the machine, which sends ever more painful electric shocks into Winston's beaten, broken body. O'Brien asks the same question several times and each time receives the same answer, with the same result for Winston: more excruciating pain. O'Brien wants Winston to realize that, from his perspective as an individual man, he cannot know the truth; only the Party can. Whatever vestiges of reality exist within Winston's mind must be expunged completely, to be replaced by the alternative reality inhabited by the Party.