The side bar on the right usually displays some interesting stuff. For example community bulletin, hot network questions, recent badges etc. On some pages some chat rooms are shown in the side bar. They might look like this:
What are the criteria based on which rooms shown there are selected?
From what I have observed I would guess that: Only rooms which had some activity not too long ago are shown there. They are selected from the
rooms associated with the site you are currently on. (
I don't think I have seen a room associated with a meta site there.) I don't recall whether I have seen a room imported from comments in the side bar.