The prologue which opens Shakespeare’s Romeo and Juliet provides a brief exposition of the play. The Chorus tells the audience that the play is set in Verona, Italy, and that the plot involves two feuding “households," or families, “both alike in dignity,” meaning of equal high stature.
More specifically, the prologue advises the audience that the plot of the play focuses on “a pair of star-crossed lovers” whose “misadventur'd piteous overthrows,” meaning that their attempts to overcome the hatred between their families will result in their deaths.
More than simply an introduction to the play, the prologue is a promise that Shakespeare makes to his audience. Shakespeare promises that, during the course of the play, the audience will see the events occur that are revealed in the prologue.
Most of the people in the audience for Romeo and Juliet would know the story of the ill-fated lovers Romeo and Juliet, and some would be familiar with the poem The Tragicall Historye of Romeus and Juliet, published in 1562, which is one of the sources for Shakespeare’s play of Romeo and Juliet. In the prologue, Shakespeare implies that his version of the story will be different and better.
The Chorus also makes a promise to the audience that anything that’s been left out of the prologue will be shown or explained in the play, which heightens the audience's anticipation for the play.
CHORUS: The which if you with patient ears attend,
What here shall miss, our toil shall strive to mend.
Notable differences between the prologues of other Shakespeare plays and the prologue to Romeo and Juliet are that the prologue in Romeo and Juliet is much shorter, at only fourteen lines, and significantly more succinct than all of the other prologues, and the prologue in Romeo and Juliet is in the form of a sonnet. This raises the tone of the play and prepares the audience for the tragic story of the “star-crossed lovers” that follows.