Winston spends an undocumented amount of time in the ironically named Ministry of Love, being tortured and starved in order to brainwash and mold him to be a servant to the Party. They had brought him there after discovering his intent to rebel against the party when he revealed his hatred to the party to O'Brien. They tossed him in a room and from that point on, had manipulated, questioned, yelled, hit, tortured, beat and starved him into almost submission. It was a long, drawn-out process, as Winston's mind and logic kept stepping in, keeping him from total subservience to the Party. At one point, O'Brien allows Winston to look at himself in the mirror, possibly as another way to discourage Winston from any form of independence or strength. What Winston sees shocks him:
A bowed, gray-colored, skeletonlike thing...the creature's face seemed to be protruded...a nobby forehead running back into a bald scalp, a crooked nose and battered-looking cheekbones...the mouth had a drawn-in look...truly frightening was the emaciation of his body. The barrel of the ribs was as narrow as that of a skeleton; the legs had shrunk so that the knees were thicker than the thighs.
The description goes on; Winston barely recognizes fragments of himself in the image, and instead thinks the person before him is near 60, and suffering from a terminal illness. But no, it is him. It is what the Party has done to him. It is horrifying not only to him, but to the reader, to think that would have been done to another human being, all in the name of power and submission. This glimpse of himself is one of Winston's last moments of strength; after he sees himself, he caves. He gives in. He submits to the party, for the most part. That image was so shocking that it served O'Brien's purposes: it made Winston submissive and pliant, a workable servant for the Party.
I hope that those thoughts helped a bit; good luck!