The quote is from Part 2, chapter IX
At the beginning of the Hate Week, Oceania is at war with Eurasia. After six days of forced mass demonstrations where people shouted anti-Eurasia slogans, daily broadcasts furiously expressed hate for Eurasia and crowds are worked up into deep hate of Eurasians, it is suddenly announced that "Oceania was not after all at war with Eurasia. Oceania was at war with Eastasia. Eurasia was an ally.
Our hero, Winston, needs to alter and erase from existence all documents mentionning there had ever been a war with Eurasia. Here is the slightly larger quote:
"Oceania was at war with Eastasia: Oceania had always been at war with Eastasia. A large part of the political literature of five years was now completely obsolete. Reports and records of all kinds, newspapers, books, pamphlets, films, sound-tracks, photographs — all had to be rectified at lightning speed. Although no directive was ever issued, it was known that the chiefs of the Department intended that within one week no reference to the war with Eurasia, or the alliance with Eastasia, should remain in existence anywhere. The work was overwhelming, all the more so because the processes that it involved could not be called by their true names."
What is meant by this is that Winston (or anyone else in the Ministry of Truth) cannot simply admit to anyone that what they are doing is falsifying evidence. This is a very good example of what the book refers to as 'doublethink'.
Doublethink is the act of ordinary people simultaneously accepting two mutually contradictory beliefs as correct: we accept that we need to modify the paper to make it say "at war with Eastasia", instead of "with Eurasia"; then, as soon as the deed is done, we accept that the paperhad always said"at war with Eastasia" and we can point to it as evidence that there has never been such a correction.
In Part 1, chapter 3 you can find this definition of doublethink: "to tell deliberate lies while genuinely believing in them, to forget any fact that has become inconvenient, and then, when it becomes necessary again, to draw it back from oblivion for just as long as it is needed". Doublethink explains why the true names of the processes (data falsification, mystification, lying) cannot be uttered out loud or, for that matter, recognised.