This part of the play comes in Act I just after the Reverend Hale has arrived in Salem to offer his expertise to sort out what has been happening. When Mrs. Putnam accuses Tituba of being able to conjure, she needs to offer some form of proof to back up her claim. When she does so, it is proof that she herself has contacted Tituba to ask her to perform a spell so that she can know who killed her seven babies who all died. Note what she confesses to:
I sent my child--she should learn from Tituba who murdered her sisters.
Rebecca Nurse responds in horror to this, as what Mrs. Putnam did was to send her daughter to commune with the dead spirits of her sisters, who all died in childbirth. Mrs. Putnam therefore contacted Tituba before in order to try and learn why her daughters all died.
Mrs Putnam has had seven babies die before they were baptized and although now we would associate that with a medical infertillity problem, back then they blamed the devil. So she blamed 'witches' like Rebecca Nurse for this.
After her only living daughter Ruth ends up in a similar condition to Betty Nurse, she tries to talk to Tituba to discover why her babies die and if she has the power to stop it.