Mary Warren brings Elizabeth Proctor a doll. Referred to as a "poppet" in the play, the doll turns out to be part of Abigail's plan to accuse Elizabeth and get her "out of the way" so that she can re-kindle her affair with John Proctor. Abigail's plan works insofar as it succeeds in getting Elizabeth arrested.
Proctor is therefore not being entirely metaphorical when he says:
“the little crazy children are jangling the keys of the kingdom, and common vengeance writes the law!”
The second act of the play explores the extent to which Abigail and other accusers are given credence while those accused are denied credulity and denied justice.
Mary Warren brings the doll into the Proctor home after spending a day at court, sitting near Abigail. Later that same night authorities arrive at the Proctore home when Reverend Hale is also present. A story emerges that Abigail has become sick at the dinner table with stabbing pains in her belly.
Abigail has accused Elizabeth Proctor of witchcraft on/against her. The authorities then come looking for a doll at the Proctor home and find the one that Mary Warren has brought into the house.
The doll, it turns out, has needles stuck into its belly beneath the dress. This is taken as evidence against Elizabeth and she is taken to jail.
The poppet represents the lengths to which Abigail is willing to go to seek vengeance. It is now clear that the doll was planted in the house by Mary Warren. Abigail has stuck her own belly with a long needle. (eNotes)