Doublethink is a way for you to think one way but believe another. It is protection for anyone in the Party who does not truly believe the policies.
For instance, when Winston is being tortured by O'Brien, O'Brien tells Winston that two plus two equal five. Winston oberves that O'Brien gives absolutely no indication that he believes any other thing than that 2+2 actually DOES equal 5. Even though Winston knows this is not true, he also eventually begins to think that 2+2 MIGHT equal 5.
It's like thinking one thing and saying something completely opposite to hide your true feelings...doublethink is incredibly wearisome and difficult to keep up for any period of time, especially in a society like Oceania where people are constantly looking for a reason to accuse someone else of thoughtcrime.
Double think is to hold to condradictory meaning at the same time and to still believe in only one.. for eg. The party says 2+2=5 and you know that that 2+2=4 but you still beleive that 2+2=5 as the party says so...
even to understand doublethink the use of doublethink is needed.
THe opposite meaning that cancell each other out !! Doublethink is also known as reality control..
it makes people follow the partys rules and regulations without questioning their methods...