TL:DR - this is a plugin that lets users collaborate on a native OSX design app called Sketch by reading/writing files on any synced filesystem (google drive, dropbox, sync, etc.).
It's essentially google-docs style collaboration (multiple users in a single doc), but doesn't require a dedicated server and has to get around using all the diff/match/patch magic you can apply when working mainly with text (since Sketch uses lots of binary files).
What's particularly cool is making an existing app collaborative by simply creating a plugin and leveraging realtime syncing services. Curious how many other apps could use this same approach. :)