In Part One, Chapter One, Winston describes the Two Minutes Hate. As this event draws to a close, the face of Big Brother appears on the telescreen followed by the three slogans of the Party:
WAR IS PEACE
FREEDOM IS SLAVERY
IGNORANCE IS STRENGTH
These slogans are paradoxical but demonstrate the extent of the Party's power. The Party is so powerful, for instance, that it can make people believe that two opposing and illogical ideas are one and the same.
Furthermore, it is significant that these slogans appear at the end of the Two Minutes Hate. On a practical level, they bring the Hate to an end, functioning like a calm after the storm. Looking deeper, however, they make people feel safe and protected by the Party. We see this most strongly in the description of the "little sandy-haired woman" who calls out "My saviour!" when she sees Big Brother's face and the slogans on the telescreen.