It's the opposite of an epilogue. Shmoop out.
Just kidding. Shmoop would never do that to you.
A prologue is a section that comes at the very beginning of a piece of literature, and it helps us figure out what's to come. It might give us the scoop on the background of our characters or clue us into the setting. It might hint at themes, or even give us a warning. Prologues, like epilogues, are written in the voice of the text itself.
In drama, the prologue comes in the form of a small speech addressed to the audience before the start of the action of the play. It's spoken by one of the actors, sometimes in character, sometimes not. The prologue preps us for what's about to go down, like the prologue from Shakespeare's Romeo and Juliet (which, we should point out, is probably the most famous prologue of all time):
Two households, both alike in dignity,
In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean.
From forth the fatal loins of these two foes
A pair of star-cross'd lovers take their life;
Whose misadventured piteous overthrows
Do with their death bury their parents' strife.
The fearful passage of their death-mark'd love,
And the continuance of their parents' rage,
Which, but their children's end, nought could remove,
Is now the two hours' traffic of our stage;
The which if you with patient ears attend,
What here shall miss, our toil shall strive to mend.
Yeah, we're gonna go ahead and let that one speak for itself.