What does Mary Warren bring home to Elizabeth Proctor?

The part of the play that you are asking about is in Act 2.  Mary Warren has been at the court proceedings all day, because she is one of the "witnesses" to witchcraft.  She comes home late that day, and John Proctor isn't too happy about it.  Mary sort of side steps John by going to Elizabeth and giving her a present.  The item that Mary Warren brought home to Elizabeth is a "poppet."  

Mary Warren: I made a gift for you today, Goody Proctor. I had to sit long hours in a chair, and passed the time with sewing.

Elizabeth, perplexed, looking at the doll: Why, thank you, it's a fair poppet.

A poppet is a children's doll.  The present is completely innocuous, and Mary has no ill intents with giving it to Elizabeth.  The poppet becomes important a bit later in the act, because Abigail Williams was using the doll to spoof the court into thinking it is used for some kind of voodoo witchcraft.  Earlier in the day, Abigail Williams stuck a needle into the doll and then secretly stabbed herself with a different needle.  That was enough evidence to accuse Elizabeth of intending witchcraft harm against Abigail.  

Cheever, his hands shaking: Why, this go hard with her, Proc-tor, this - I had my doubts, Proctor, I had my doubts, but heres' calamity. To Hale, showing the needle: You see it, sir, it is a needle!

Hale: Why? What meanin' has it?

Cheever, wide-eyed, trembling: The girl, the Williams girl, Abi-gail Williams, sir. She sat to dinner in Reverend Parris's house tonight, and without word nor warnin' she falls to the floor. Like a struck beast, he says, and screamed a scream that a bull would weep to hear. And he goes to save her, and, stuck two inches in the flesh of her belly, he draw a needle out. And demandin' of her how she come to be so stabbed, she - to Proctor now - testify it -were your wife's familiar spirit pushed it in.

Proctor: Why, she done it herself! To Hale: I hope you're not takin' this for proof, Mister!

Hale, struck by the proof, is silent.

Cheever: 'Tis hard proof! To Hale: I find here a poppet Goody Proctor keeps. I have found it, sir. And in the belly of the poppet a needle's stuck. I tell you true, Proctor, I never warranted to see such proof of Hell, and I bid you obstruct me not, for I -