In Act Three, Mary Warren's testimony, if believed, will strike down all of the previous accusations, because it will show that Abigail and all of the other girls are lying. All of the women and men that have been accused up to this point, have been accused by these girls. Mary is going to testify that they were all lying, pretending, faking, and getting caught up in the emotion of the moment. If the girls were lying, then these women weren't witches at all, but instead just victims of the girls' lies. It would mean that all of the indictments would have to be overturned, and the women set free--including John, Giles and Francis' wives.
Mary's testimony strikes at the heart of the entire scandal behing the Salem witch trials. It will assert that Abby and her cronies are purposefully sending women to their deaths; in essence, that they are murderers. It would also overturn the rulings of the judges, making all of the judgments they deemed seem evil and murderous also. So, it is a very important testimony, indeed. Too bad she doesn't stick to it, huh? I hope that helped; good luck!