In this scene, Mary has just told everyone in the house that the poppet they found there was indeed hers, and that she sewed it in court that day. She adds, "I-I believe I did, sir" when asked if she was the one to put the needle in it. Even better, she says, "Ask Abby, Abby sat beside me when I made it." So, if this is true, and Abby saw her put the needle in the poppet's belly, then that means that Abby stabbed a needle into her own belly, and then accused Elizabeth Proctor of doing it, through her spirit as a witch. That means that Abby was plotting Elizabeth's murder. Which she indeed was; Elizabeth already suspected it. Earlier in that act Elizabeth had stated ominously and incitefully, "She wants me dead, John, you know it!" Unfortunately, no one but John (and Elizabeth) really believes that Abby is capable of such malicious intents, and Elizabeth is wisked away to jail, all because Abby was an opportunistic plotter who schemed to get here there.