In the ancient Greek theater, the chorus was a rowdy bunch. They were the peanut gallery that sang, danced, and made comments about the action on stage. They also gave you key information about stuff happening off stage—like battles and sea voyages.
While you don't see the traditional Greek chorus often in modern theater, the term can sometimes be used to refer to a character that gives the audience key information or offers commentary, like Hurley on Lost. By the way, we're being totally serious.