You can find a pretty long description of the Two Minutes Hate in Chapter 1.
But pretty much what is going on is that all the people get together at some specified time and the vent their hatred at someone for two minutes. In the Two Minutes Hate that we see in this chapter, they are supposed to be angry at Goldstein (he's always in them) and at the rows and rows of Eurasian soldiers that they see marching on the screen.
Basically, the people just completely freak out. They get to the point where they are jumping up and down screaming at the screen and someone even throws a book at the screen. As the book says:
In its second minute the Hate rose to a frenzy. People were leaping up and down in their places and shouting at the tops of their voices in an effort to drown the maddening bleating voice that came from the screen.