In The Crucible, Mary Warren is a girl working for the Proctors, who is terrified of her friend Abigail, and the power that Abby has in court. By the end of Act Two, it has become terribly apparent that whatever Abby says is believed in the courts; if she accuses someone of witchcraft, that person is arrested. Elizabeth Proctor referred to Abby's power earlier when she mentioned that in town, "where she walks, the crowd will part like the sea for Israel." This is a reference to the Bible where Moses parted the Red Sea for the Israelites to pass unharmed; essentially, Elizabeth is alluding to the fact that people consider Abby to be an angel sent from God, and revere her as such.
So, when John Proctor tells Mary that she needs to go to the courts and let them know that Abby saw her sew the doll and put it under her chair, Mary is naturally afraid. She is worried that Abigail will retaliate and accuse HER of being a witch. She repeats over and over at the end of Act Two, "they'll turn on me," meaning Abby and the other girls. It is a frightening thing for her to go to the courts and basically say, "Hey everyone. Uh, Abigail made up the story about the doll to get Elizabeth arrested." Mary is quite aware of the potential for Abby to turn and declare Mary herself to be a witch. Her life is at stake, quite literally.
I hope that those thoughts helped; good luck!