Elizabeth wants John to go to the town and tell them that Abby said "it had naught to do with witchcraft", referring to all of the accusations that were going on. She knew Abby had told him the girls had just gotten scared, and so told John that "God forbid you keep that from the court, John. I think they must be told." She hoped that once people knew Abby was a fraud, the insane accusations would stop.
There are a couple problems with this plan. 1. Abby is considered almost a saint in the town now, so proving her a fraud will be a tricky and difficult business. 2. John and Abby were alone when she told him, so he has no witness but his word. In the end though, Hale shows up and they tell him, so the word does gets out (not that it does much good). I hope that helps!
Elizabeth wants John to go to Salem and say the girls were playing in the woods.