Microsoft released this Windows 10 Insider Preview build 14931 on September 21, 2016. It was released in the late evening Eastern time. This build brings no visible new features to the operating system but does include plenty of under-the-hood changes. As part of the preview, Microsoft is also updating their preinstalled apps with new features for insiders to test out. These features are not tied to this build or Redstone 2 in particular. They are app updates that can, and likely will, roll out to users of the stable branch (Redstone 1). Microsoft is using the Insider Program and releasing the apps exclusively to Redstone 2 users for now to test out the new feature before rolling it out broadly.
Windows 10 Redstone 2 is the next feature update to Windows 10. It is slated for release in spring of 2017, according to various reports. This will make Redstone 2 the third major update to Windows 10, after Threshold 2 and Redstone 1.