Mary Warren tells the court that this entire time, all of the girls that have been accusing people of witchcraft have been faking it. She tells Danforth and the other judges that "it were all pretense." This is a HUGELY serious claim, one that, if true, would undermine the authority of every single conviction of witchcraft. Everyone that had ever been accused of being a witch would have to be let go. It is also a rather unbelievable claim to the judges--why would a group of girls purposely send other people to their deaths? That's preposterous. Plus, as Parris points out, when they all screamed and fainted, it seemed so real. He says, of the times that Mary fainted,
"But you did turn cold, did you not? I myself picked you up many times, and your skin were icy."
They are having a hard time grasping the idea that Mary could fake fainting and being icy cold. It just seems to far-fetched.
Mary tries to explain by saying that she saw everyone else freaking out, and it was easy to pretend, and even to believe that she was terrified herself. This makes sense, especially if you have ever been in a large crowd of people that starts screaming; it's a bit contagious. Unfortunately, the judges don't believe Mary's claim, and Abby and the girls get away with it all. I hope that helps; good luck.