The Chorus, at the beginning of the play, functions like a Prologue. He/she sets the stage for the events, giving background to the feud and also telling the audience that Romeo and Juliet will settle the feud by killing themselves, altering the audience that Fate is in control of these "star-crossed lovers."
He/She appears again at the beginning of Act II to, dramatically, provide a transition between the Capulet feast that ends Act I and the scene in Capulet's garden between Romeo and Juliet at the beginning of Act II. He/she focuses the audience's attention on the fickleness of Romeo and the part that chance plays in providing them this occasion to meet and express their love for each other.
The Chorus, after this, disappears from the play, something that many people find odd. This disappearance also leads many theatre companies to cut The Chorus from the play, having his text in Act I spoken by the Prince.