What is an objective summary of the Prologue in Romeo and Juliet?

The Prologue to Shakespeare's Romeo and Juliet simply tells the audience what will happen in the play. It immediately reveals the ending by telling the audience, 

A pair of star-crossed lovers take their life

Before that, it informs the audience that the Montagues and Capulets are equal in their importance in Verona ("Two households both alike in dignity"). They are involved in a longtime feud. Shakespeare never tells us why they are fighting but the feud is "ancient" and sometimes the innocent citizens of Verona are also caught up in the violence ("civil blood"). It proclaims that the Montague and Capulet children will be involved. They are "star-crossed", indicating fate has conspired against them, and their deaths are pre-ordained. Only their demise will ultimately end the rivalry ("Which, but their children’s end, naught could remove").

In the final three lines Shakespeare tells us it will take two hours for this story to be told (a dubious claim since it takes at least three if everything is included). He indicates the audience should listen closely ("with patient ears attend") to his story. This is important because Shakespeare's story is not so much about plot, but about the words he uses to spin his tale. Moreover, he tells the audience if there's anything he's missed in the  Prologue, he will work hard to fill in those gaps in the overall telling ("our toil shall strive to mend"). 

The Prologue is also a Shakespearean sonnet in its construction. It is written in iambic pentameter (five stressed and five unstressed syllables per line) and rhymes ababcdcdefefgg. Fourteen lines with alternating rhymes and a couplet at the end.