After having a conversation in the canteen with Syme, Winston has a flash of insight in which he realizes Syme will be vaporized because he's too intelligent. Their canteen conversation had been about the Newspeak dictionary Syme was working on. Syme had waxed enthusiastic about reducing the number of words in the language to the fewest possible. Syme had grasped that reducing people's vocabulary to a bare minimum reduced people's capacity to have subversive thoughts, and he talked about this knowledge openly with another person. In other words, he grasped the big picture in a way that violated the rules of doublethink, in which he was both to know and simultaneously remain unaware of what he was doing. Syme ends up disappearing, almost certainly vaporized. O'Brien, apparently aware that Winston has talked to Syme, uses a mention of this "friend" of Winston's (since Syme has been vaporized O'Brien can't use his name) to suggest to Winston that he, O'Brien, might be a conspirator against the regime.
O'Brien reveals a secret when he gives Winston a copy of The Book, laying bare the logic of Oligarchic Collectivism. The Book confirms what Syme had said about dumbing down the language: The Book reveals the importance of crimestop, or stopping any subversive thought before it can even occur. Syme clearly did not practice crimestop and, like Winston after reading The Book, he knew too much to be allowed to continue unchecked.