At the end of Act III, Romeo has been banished because of the murder of Tybalt and Juliet's father has ordered her to marry Paris. Juliet tries to argue with her father but fails.
Once Capulet leaves Juliet to herself, the Nurse approaches her to urge her to forget Romeo and accept Paris as her new husband. Juliet doesn't argue with the Nurse, understanding that the household is against her. She tells the Nurse that she (Juliet) will go to the Friar to ask forgiveness for upsetting Capulet.
In the last speech of this scene, the Nurse has just left Juliet to plan Juliet's trip to the Friar. Juliet, speaking to herself, says the following:
Ancient damnation! O most wicked fiend!
Is it more sin to wish me thus forsworn,
Or to dispraise my lord with that same tongue
Which she hath prais'd him with above compare
So many thousand times?--Go, counsellor;
Thou and my bosom henceforth shall be twain.--
I'll to the friar to know his remedy;
If all else fail, myself have power to die.
What this means is this:
Juliet curses the Nurse for insulting Romeo, calling the Nurse a wicked and evil woman. She announces that her relationship to the Nurse is over. She further announces that she will go to the Friar to seek help. She finishes by saying that, if no help is available, she wil kill herself.