The Prologue to Romeo and Juliet serves the same purpose of the Chorus in Greek Tragedy. In Tragedy, there is no escape -- the story is known, and everyone must yield to their particular fate. The Prologue actually summarizes the play and tells what will happen -- the children of these two opposing houses will die, and "bury" their parent's strife. It doesn't "give the play away," rather, the brief description causes the listener to want the full details found in the production of the play.
The Prologue is actually in a sonnet form, 14 lines of iambic pentameter with a rhyming couplet at the end. The first 4 lines describe the setting, the next the complications and tragedy of Romeo and Juliet's death, and the last 4 the insight that the feud could only have been resolved by the tragedy. The final couplet hopes that all in the audience will comprehend the story.