what do abigal, betty, mercy and mary discuss after reverend parris leaves his daughter room?

They discuss how to get their story right. Abigail threatens all of the girls to do and say what she tells them to or she will hurt them.

Now look you. All of you. We danced. And Tituba conjured Ruth Putnam's dead sisters. And that is all. And mark this. Let either of you breathe a word, or the edge of a word, about the other things, and I will come to you in the black of some terrible night and I will bring a pointy reckoning that will shudder you.