Today, Microsoft published Windows 10 SDK Preview build 17025 on its Insider download page. There's been no official announcement regarding what's new, so all we really know is that you'll be able to target build 17025 with your apps.
It's the first SDK Preview build for Redstone 4, which is the codename for the next feature update to Windows 10, landing in the spring of 2018. The build number aligns with the current Fast ring build that was released last week, so perhaps this means that there might finally be a Redstone 4 Slow ring candidate.
Obviously, it's not recommended that you run this SDK in a production environment. For that, you should stick with the Fall Creators Update SDK. The Preview is mainly for if you want to start testing out some of the new APIs that will be coming with Redstone 4.
If you want to install the SDK on top of your Visual Studio 2017 installation, you can find it on the Insider downloads page here.
Update: Microsoft has finally published a blog post for the SDK Preview build. One thing that you'll want to be aware of is that installing the SDK on a non-Insider machine is not supported, and it might fail.