What is the Two Minutes Hate and what is its purpose in the story?
The Two Minutes Hate is a daily ritual performed by the members of the party in the book's dystopian society. During the Two Minutes Hate, the party members watch films of people like Goldstein who are enemies of the Party. They scream in hatred at these people.
The purpose of this is to help make the people lose their individuality. They are all supposed to show the same emotions about the same things at the same time. It makes them feel that they and everyone else all feel the same -- and that these feelings are what the Party tells them to have.
The Two Minutes Hate is a ritual observance that is designed to use the collective rage of the people against supposed "enemies of the Party" to strengthen the Party's position among the people. The ritual serves to unify the people in the observance of the ritual.
The Two Minutes Hate also serves the purpose of religious observance by deifying Big Brother. It serves to channel the rage that persons may feel toward the lack of control over their own lives away from the Party and against purported enemies of the Party. Because Big Brother is proclaimed to be benevolent and good, any enemy is automatically evil and bad. While it is a "two minutes hate" the desired end result is less hatred of Big Brother.
This is very similar to what some politicians do to strengthen their position when they feel threatened. The politicians will invent an enemy and then channel the public rage against that enemy.
The two minutes hate was a time when people got together every day and screamed at videos of people opposed to big brother. it was like a bandwagon moment, when everyone was doing it, you began to do it too, it was in a very effective way a brainwashing session, to allow them to think about nothing but how this person had betrayed big brother, forming a negative opinion, without even getting the entire story (*ahem* somewhat sounds similar to today's media*)
This serves as a way to unite the public against a common enemy and therefore the Party's position will not be threatened.