Abigail tells Proctor that Betty wasn't bewitched, or a witch herself, but that she had "only gone silly somehow". She said "we were dancin' in the woods last night, and my uncle leaped in on us. She took fright, is all." This is significant news, considering later when she is in danger of getting in trouble, she contradicts this claim by "confessing" to trafficking with the devil, and then naming women in the town as witches.
Later, John tells his wife what Abby had said, but leaves out the little detail that he and Abby were alone when this information was shared. Elizabeth wants John to tell the courts, thinking that this will stop the madness, because it proves that the girls were not bewitched. Unfortunately, it doesn't work.
When John and Abigail are alone, the audience sees by the way they speak that they did have some kind of affair. Elizabeth, Proctor's wife, discovered the situation and dismissed Abigail. But Abigail is still holding out hope that John will leave Elizabeth for her. In addition, Abigail also reveals that the accusations of witchcraft are untrue. She implies that all the girls were doing was dancing in the woods and that Betty is probably faking in order to avoid punishment for dancing, not witchcraft.
Well actually there is an Abigail and Proctor scene in Act 2. It is a scene that is rarely used. The scenes that the other people answering this question mention occurs in Act 1 which is what I'm sure you were referring to but it would be Act 1 Scene 2 not Act 2.
In Act 2 after Elizabeth has been taken to jail there is a scene in which Abigail and Proctor meet in the woods. At the end of the scene Abigail says "Fear naught. I will save you tomorrow. From yourself I will save you." in response to John Proctor's claim that he will admit to lechory in the court in order to stop Abigail and free Elizabeth.