Today, Microsoft released a new build of Windows 10 to Fast Ring Insiders - Build 16257. Right after that, they made a gift for Slow Ring participants of the Windows Insider program. Windows 10 build 16251 is available for everyone who has configured their OS to receive updates from the Slow ring.
Windows 10 Build 16251 is a preview version of the upcoming Fall Creators Update, version 1709. It was recently released to the Fast Ring.
For everyone on the Slow ring, Windows 10 Build 16251 will bring a number of new features. A "link your phone and PC" feature, Cortana voice commands to lock, sign-out, shutdown, or turn off your PC and a bunch of improvements are included in this build.
As it is the Slow ring, we can expect the set of official ISO images pretty soon.