It's intended for design/implementation discussions which are bigger than a single github issue or pull request, but which aren't relevant to library users.
It's readable to the public, but in order to keep it focused and non-spammy, it's invitation-only for joining in order to be able to post/comment. At the moment that's just committers and GSoC students/mentors. If you want to participate in discussions there, send me an email requesting an invitation.
(Note that long ago, there was a [hidden email], but we stopped using it, and we won't be resurrecting it.)