What information does John Proctor reveal to Reverend Hale? What does the Reverend want John to do with the information?

The answer to this question can be found in act 2. Specifically, look to the part when Hale asks John to name all Ten Commandments. John successfully names nine, but he draws a blank on adultery. Elizabeth comes to his aid, and Hale seems mostly satisfied. He turns to leave, and Elizabeth says that John must tell him (Hale). She says this loudly enough for Hale to hear and turn back around.

Elizabeth, with a note of desperation: I think you must tell him, John.
Hale: What’s that?
Elizabeth, restraining a call: Will you tell him?
Slight pause. Hale looks questioningly at John.

John then, "with difficulty," admits that Abigail Williams told him that the children's sickness has nothing to do with witchcraft. The girls were "sportin' in the woods" when Mr. Parris startled them.

Proctor, with difficulty: I—I have no witness and cannot prove it, except my word be taken. But I know the children’s sickness had naught to do with witchcraft.
Hale, stopped, struck: Naught to do—?
Proctor: Mr. Parris discovered them sportin' in the woods. They were startled and took sick.
Pause.
Hale: Who told you this?
Proctor, hesitates, then: Abigail Williams.
Hale: Abigail!
Proctor: Aye.
Hale, his eyes wide: Abigail Williams told you it had naught to do with witchcraft!

John then clarifies when Abigail told him this information, and Hale demands to know why John hasn't already spoken up about all of this. John Proctor has done a good job of trying to stay out of Salem's business, and he admits that he was unaware that the witchcraft business had gotten so completely out of control until now.

Hale, suspiciously: Why—why did you keep this?
Proctor: I never knew until tonight that the world is gone daft with this nonsense.

John Proctor candidly reveals to Reverend Hale that he has wondered if there are actually any witches in this world. More specifically, he doubts whether there are any witches in Salem. This is a roundabout way of saying that the whole witch-craze is based on nothing but a lie and that Abigail Williams's accusations of witchcraft are a complete fabrication. Hale tries to pin down Proctor's precise beliefs as to the existence of witches. Sensing a trap, Proctor simply replies that he has no knowledge of them, but as the Bible speaks of witches he will not deny them.
Proctor also reveals, at Elizabeth's urging, that Parris caught Abigail and the other girls frolicking in the woods that night. He also mentions that Abby frankly told him that the girls' sickness had nothing to do with witchcraft. Hale may be a zealous witch-hunter, but he actually wants to get at the truth. He believes in due process and the rule of law, and so he wants Proctor to testify in court, and repeat what he's just told him.