What is “reintegration”? What is the second phase of “reintegration for Winston?

Reintegration is brainwashing. After Winston was tortured, he had to go through three phase of reintegration that would prepare him to return to society.

The second phase of reintegration in to make the patient understand why the Party exists. People were taught that the Party existed for the good of everyone, that man by himself is weak. Winston didn't go along with this and continued to believe that the party would be overthrown. O'Brien ruins this belief when he places Winston in front of the mirrors.