One major theme of Romeo and Juliet has to do with both love and hate and how, frankly, alike they are in terms of their effects on the lives of those who feel such intense and passionate emotions. Both love and hate often cause people to behave impulsively and can lead to tragic consequences (as we see in the play itself). The four lines, "The fearful passage of their death-marked love / And the continuance of their parents' rage, / Which, but their children's end, naught could remove, / Is now the two hours' traffic of our stage," seem to best embody this theme (9-12). These lines refer to the progress of the love between Romeo and Juliet, the love that was destined to end in their tragic deaths, as well as the hatred and anger that exists between their families, and, finally, how nothing could put an end to this feud except the families' shared grief over the deaths of their relatively young children.
Much of the action of the play is included within the prologue. If I were to choose one line, however, that encapsulates most of the action, I would choose, "From forth the fatal loins of these two foes / A pair of star-cross'd lovers take their life." These lines get at the heart of the action of the play. The "fatal loins of these two foes" sets up the central idea of the feud, which is at the heart of the play. Without the feud, there would be no reason for Romeo and Juliet not to be together. The reference to "star-cross'd lovers" speaks to their courtship and eventual marriage. Likewise, the note that they "take their life" portrays the action in the later acts and the ultimate impact of the play as a whole.
Let us first take a closer look at what the question is actually asking. We're looking for an image, phrase, or line from the prologue that sums up the larger "MESSAGE" or "THEME" of the play -- not the EVENTS of the play. This is an important distinction.
So while lines like, "From forth the fatal loins of these two foes/ A pair of star-cross'd lovers take their life" effectively sum up the plot of the play, they don't really suggest what we're meant to learn or take away from it in a broader sense.
Indeed, most of the prologue functions simply as backstory and plot summary. Shakespeare is telling the audience upfront what's going to unfold on stage in order to pique their interest and set an appropriately compelling and tragic tone for his play. And so, for the most part, the prologue merely sets the scene, provides some initial context, and summarizes the events of the story to follow.
Now, returning to the question at hand, I would argue that the closest Shakespeare comes to elucidating a central message or overall theme is when he notes that Romeo and Juliet "Do with their deaths bury their parents' strife." Here, Shakespeare goes beyond simply describing where, when, and how the story will unfold -- rather, he's making a more sophisticated point about the tragic irony contained within it.
It's important to note that any discussion about the "central message or major themes" in a literary work will generally revolve around concepts, not characters. Shakespeare's work is considered timeless because it addresses larger, universal, abstract concepts and dichotomies underlying the human experience -- concepts the remain relevant even today. Humankind continues to confront issues of mercy and justice, good and evil, war and peace, wealth and poverty, etc.
In Romeo and Juliet, the larger theme involves the battle between love and hate. When Shakespeare notes that Romeo and Juliet "do with their deaths bury their parents' strife," he's calling our attention to a fundamental irony: Romeo and Juliet's love (and the death that resulted from it) was, in the end, the only thing that could finally put an end to years of hatred between their families. Whereas Shakespeare spends much of the prologue setting the scene and describing the story, in that particular line, he is providing a larger insight into the fundamental battle upon which the entire play is based -- the battle between love and hate.