The full quotation from Romeo and Juliet by William Shakespeare is:
Is it e'en so? Then I defy you, stars!
Thou know’st my lodging. Get me ink and paper,
And hire post horses. I will hence tonight.
Romeo has had a dream that Juliet finds him dead and brings him back to life with a kiss. Dreams, in Shakespeare's period, were considered omens, much like astrology. Thus Romeo is metaphorical associating the predictive power of dreams with the predictive power of stars. Since the stars were connected to what they predicted by laws of sympathy, and expressed divine will, to act against omens was a form of defiance. When he hears of the death of Juliet, he decided to break divine law (the law against suicide) as well as go against the prophetic dream. Had he obeyed the dream rather than defied it by poisoning himself, the play would have ended happily.