What does the Prologue add to the play???

In Shakespeare's Julius Caesar, Shakespeare continues a Greek technique and tradition by using a chorus in his play.  A chorus traditionally supplies commentary on the action of the play, interpreting the play from the perspective of conventional wisdom or common sense.

Shakespeare's Chorus does this in Romeo and Juliet.  The Chorus gives "The Prologue," and introduces the opening conflict in the play--the "ancient grudge," or feud between the Montagues and Capulets.   

The Chorus also briefly outlines what the audience will witness in "...the two hours' traffic of our stage."  The "star-crossed" lovers and their "death-marked" love are introduced, as is the loss of their lives.  This establishes the tragic mood of the play, as the doomed lovers meet and fall in love and are prohibited from being together.