Cinder,I think (if memory serves at all), that it's a chant, a choric representation/response, or even a song or chorale, & its performers, that tells at least part of the narative/sets up the action. It was(I think) done w/various numbers of singers stationed somewhere in the audience or positioned elsewhere in the auditorium. Don't remember if it was immediately in front, or midway back, or all the way to the back or where. Or even if it possibly changed depending on what play was being given. Sorry.

This was before the idea/definition of a "play" included multiple actors being onstage at once or lots of entrances & exits.
It was (don't quote me on this, it's been awhile since I've dealt with ancient theatre & lit) sorta a "call-and-response" version of dialogue. Otherwise the performer onstage would have been doing endless monologes & soliloquies! heh.
Hope this helps a little.
Chyia, forgetful
Edit: Well I wasn't too far off. Just type ancienthistory.com.theater into your browser window & it should take you to a list of terms/definitions
xx Chy