The prologue of Romeo and Juliet not only sets the scene for what the audience is about to see, but also predicts the doomed ending of both Romeo and Juliet. Thus, from the outset, we know that Romeo and Juliet will die at the end.
The line you referred to is part of a larger set of 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 (Prologue, lines 9-12)
Essentially, the lines say that for the next two hours (the approximate length of the play) we will learn about the story of Romeo and Juliet's love and of their parents' anger. The 'continuance of their parents' rage' refers to that anger, an anger that has continued through a long-standing feud. The next line tells us that "naught could remove" the anger except "their children's end." In other words, only something as horrific as their children's deaths could stop the anger and and the feuding.