This is an excellent question, as one of the main themes of the entire play is that of fate or destiny and the way that the destiny that we have charted out for us can be very different from the destiny that we ourselves would like to have. References to "stars" abound in the play as they indicate the Elizabethan belief that the stars, or the planets and the influence they have over our lives, actually chart out our lives for us. Stars then represent destiny or fate.
Romeo's famous line, "I defy you stars," is uttered in Act V scene 1 after Balthasar delivers news of Juliet's death. It is an emotionally fraught point in the play, as Romeo declares his intention to "defy" the future that destiny has for him. Of course, if you look back to the Prologue, you can see that the lovers are described as being "star-crossed," indicating that destiny had planned out for them a terrible end. In spite of their best intentions, destiny decreed that they were to die.