Like Greek plays such as Antigone or Oedipus Rex, Doctor Faustus makes use of an old-school group called a Chorus. They're a group of people (or perhaps a single person) who comments on the action and gives the audience important extra scoop about the characters and the goings on.
But unlike the Choruses of Greek plays, the Chorus of Doctor Faustus has a bit part. They appear only at the very beginning, middle, and very ending of the play. We hope someone reminded them that there are no small roles—only small actors.
At the beginning, they give us the lowdown on Faustus's early years and his character, before launching into a brief synopsis of what we're about to see: how Faustus's "waxen wings did mount above his reach, / And melting, heavens conspired his overthrow" (Prologue.20-21). At the end of the play, they briefly mourn Faustus's passing before giving us a possible moral for his story.
The use of a Chorus in Doctor Faustus provides the play with a built-in reader (or viewer), someone to interpret the meaning of the play for the audience, as if we needed any help. (Sheesh, those playwrights are always telling us what to think.) Maybe it's best to think of a Chorus not as an authoritative voice, but as one point of view among many—including yours.