The Prologue in The Tragedy of Romeo and Juliet lets us know in advance that which of the following will happen in the play

The prologue tells the audience that Romeo and Juliet will "take their lives" by the end of the play.

It describes the way the families are fighting, how long the play will be, and the fate of the two 'star crossed lovers', to give an overview and preview for the audience so they know what to expect.