In The Miracle Worker, keys represent knowledge and the liberty that comes with new knowledge, particularly in regard to Helen. Throughout the story, characters often mention keys and locks, reflecting the mental state of the deaf and blind Helen.
For much of the play, Helen is metaphorically imprisoned. Despite being a bright child (she clearly understands how keys work, as is evidenced in the scene where she tries locking Annie up in a room), she has never been given a thorough way of connecting with the greater world because her family perceives her disabilities as impenetrable impediments to a full life. Even though they undoubtedly love her, they essentially see her as an animal, because they are unwilling to discipline her unruly behavior and because she cannot communicate fully with them via spoken words or writing.
The ability to understand language and communicate via sign language becomes Helen's key to the greater world. Anagnos (Annie's teacher) himself compares Helen's mind to a locked safe, one which might contain "treasure" once unlocked by Annie's teaching. Even communicating to her the concept that objects have words associated with them allows Annie to help Helen begin to understand the world. In this way, Annie's teaching becomes the key to Helen's mind.