2+2 = 5
It is the ironic reflection of the double-speak Winston had been trying to avoid internalizing his whole life. As he is dying, this is the last thing he does on earth:
Almost unconsciously he traced with his finger in the dust on the table:
2 + 2 = 5
"They can't get inside you," she had said. But they could get inside you. "What happens to you here is forever," O'Brien had said. That was a true word. There were things, you own acts, from which you could not recover. Something was killed in your breast; burnt out, cauterized out.
Winston traces 2 + 2 = 5
This confirms to the reader that he is now completely orthodox and all hopes of defeating The Party have now been diminished.
However, in the american version of the book the 5 has been omitted, they can't deal with the negative ending. Without this 5 the novel fails in that it defeats Orwell's purpose in wiriting the book.