The Mer project began as a community-led effort to keep Maemo alive. Maemo was the Debian-based OS that Nokia shipped on its Nxxx internet tablets - and it's probably still the best Linux distro ever developed for small devices.
Nokia released the N900 nearly three years ago and then abandoned Maemo, later joining Intel to develop Meego, a supposed successor to its lacklustre Moblin effort. The Mer developers, full of hope, signed up to the new project, however as it turned out Meego had more of Moblin in it than Maemo and nobody was sad to see it dropped last year.
Tizen was inaugurated to take up the reins but it seems nobody has any faith in any Intel-led mobile OS effort anymore and the Mer team have just resuscitated their project. Unfortunately for those of us nostalgic for a bit of Maemo, the project will not be resuming where the earlier venture left off, instead it will be picking up the freely-available Meego code. Also, in their manifesto they say: "Primary customers are device vendors - not end-users". So I suppose no chance of picking up a cheap Nokia N810 and putting a shiny new OS on it.