From what I have gathered it is a sharding client for the upcoming Eth 2.0 network. Specifically designed for resource restricted devices and for embedded systems. Once fully operational it would be very lightweight allowing even older generation hardware to run an ETH node.
Hope that clarifies it for you.
Okay, thanks but how does it differ from plasma? or any other sharding client being developed for the ethereum 2.0? Why is this Nimbus heralded as such a big deal by the developers?
edit-
Just moved this to discussions, it's getting buried by all the "announcements" in the altcoin announcement section.
Still looking for an in-depth discussion on this.