What does Betty reveal that Abigail did in the woods in The Crucible?
To her uncle, the Reverend Parris, Abigail denies witchcraft; she tells him that the girls danced, and that is all. However, he saw a dress lying in the grass, someone naked running through the trees, and a pot with something living in it over the fire. Mrs. Putnam tells him that she sent her daughter to Tituba, his Barbadian slave, to conjure the spirits of her dead babies. When the girls are alone, Mary Warren complains that "Witchery's a hangin' error," and insists that they confess what they did. When Abigail tells Betty that she's told Parris everything and there is no reason to be afraid, Betty suddenly awakens and says, "You drank blood, Abby! You drank a charm to kill John Proctor's wife!" Therefore, their activities in the woods did not actually center on finding out the cause of Mrs. Putnam's children's deaths—a decidedly more innocent reason to be conjuring spirits. Abigail actually wants to kill Elizabeth Proctor, and when Betty reveals this, the tension in the room -- and the play—rises sharply.
In Act I, Betty Parris tells that she and many of the other girls have been out with Tituba in the woods, trying to do magic.
What she says about Abigail Williams, in particular, is that Abigail has actually drunk some blood as part of the process of making a spell.
Betty says that Abigail did this because she wanted to kill Elizabeth Proctor. After she says this, Abigail hits her and then gets the girls together to make sure that they will all give the same account of what happened in the forest -- an account that will not get them in trouble.