At this point in the play, Mary Warren is trying to insist that all of the girls that are accusing people of being witches are faking it. She tells the judges that "it were all pretense," and tries to convince them that they all just got carried away because they were originally afraid of getting in trouble over being caught in the woods dancing and doing other things. So, judge Hathorne and Parris step up, saying that they had seen her faint away before them in court, before their very eyes, and that when they touched her in her faint, she "were icy cold." They ask her how this was possible, and when she says that she was pretending, they are incredulous, and don't really believe her. At this point, they want her to prove that she can pretend, just like she does in court. They ask her to faint, right there and then.
Mary, put under a severe amount of pressure, with the judges and all of the girls (whom she is accusing of lying) watching her, has to try to pretend to faint. While this might seem simple, the pressure gets to her, and she can't do it. The judges are exultant, thinking that they have just proved her a liar--if she can pretend in court and can't now, then she MUST have been bewitched in the court, right? Mary insists that she "has no feeling for it now," and tries to explain the concept of mass hysteria. She says that in the courtroom, all of the girls were screaming, going into hysterics, and that the judges believed them, and she just got caught up in the drama, and that is what heightened her emotions and enabled her to faint there. Anyone who has been caught up in the crowd at a concert, or screamed in a movie theater because everyone else screamed can relate to the poor Mary and her predicament.
I hope that those thoughts helped; good luck!