A Proömium is, in essence, a preface. From the Greek, it literally means "before the song". From ancient times, it has been used as an introductory chapter, or prologue. The ancient writer Homer wrote many such prologues, or songs before the song, and many of them have survived until today. The Homeric Hymns is a collection of 33 such Proömien (the plural) that invoke various Gods. The language is very similar in style and rhythm to the Homeric epics, but the purpose of the poems is quite different: they serve to exalt and invoke -- in essence, they set the stage and ask the Gods to bless the artful work that is about to appear.
It is a style that has survived over the centuries: in Latin, it is spelled "prooemium", and it simply evolved to mean preface. It is no longer an invocation of the Gods, but a preamble to the story that is to come. We use them today in our storytelling as well, in order to present our reader or audience with backstories about the characters with whom we are about to go on a journey. It is used in musical compositions as well, from classical to contemporary: in this case it would be called an Overture, which musically includes the many leit motifs that you can expect to hear throughout the course of the symphony.
In short, a prooemium or Proömium will set the stage. It is a preface, preamble, or prologue for what is to follow.