The phrase "what is past prologue" can be interpreted in several ways. One interpretation is that it refers to the events that have happened before the play begins. The Chorus briefly summarizes the long-standing feud between the Montagues and Capulets, providing the audience with the necessary background information to understand the conflict that drives the play.
Another interpretation is that the phrase refers to the play itself as a prologue to the tragic events that will unfold. The Chorus hints at the fate of the star-crossed lovers, suggesting that their story is only the beginning of a larger narrative. This interpretation creates a sense of anticipation and foreshadows the tragic events that will follow.
Overall, the quote "what is past prologue" serves to introduce the play, provide context for the conflict between the Montagues and Capulets, and hint at the tragic events that will befall Romeo and Juliet.