Abigail Williams was discharged (fired) from her job working for the Proctors. She worked for Mrs. Elizabeth Proctor but was fired. Mrs. Proctor fired her because Abigail was having an affair with Mr. John Proctor. You can see proof of this in Act I, Scene 2.
It is this firing that causes a lot of problems for the Proctors. Abigail will end up accusing both of them of witchcraft. Elizabeth will not die because she is pregnant when sentenced, but John will not be so lucky. He will be executed at the end of the play.