1 C-3PO: So Artoo, there’s this theory that we play the role of a Greek chorus in this story.
1 R2-D2: <whistle whistle beep>
2 C-3PO: A Greek chorus provides commentary or opinions about the story, for the benefit of the audience.
3 C-3PO: Essentially, we explain stuff and make comments, and the main characters ignore us for the most part.
4 R2-D2: <beep whistle buzz>
4 C-3PO: I know! Have you ever heard anything so preposterous?!

We've had a Greek chorus before, but this is a slightly more historically correct version.

As 3PO explains: A Greek chorus provides commentary or opinions about the story, for the benefit of the audience. Essentially, they explain stuff and make comments, and the main characters ignore them for the most part.

And as TV Tropes points out, R2-D2 and C-3PO are a modern example of this in action. Although the characters do get caught up in some of the action, rather than being completely detached from the story.