Microsoft has been releasing Windows Server Insider Preview builds a bit more frequently than usual for the past few weeks, and today, it's build 19613 that's being released for Insiders. As usual, the build number matches that of last week's Windows 10 build in the Fast ring.
It could be that Microsoft is getting extra time to release new builds now thanks to not sharing what's new in each release. For the third consecutive week, this build comes without a changelog, so it's unclear what has changed with this build, or any of the past three builds. Windows 10 itself hasn't gained much in the way of new features yet, so it's likely that the same applies here, but there's no way to know for sure.
In addition to the Windows Server builds, there are also matching builds of the Windows 10 Software Development Kit (SDK), Assessment and Deployment Kit (ADK), and Windows Driver Kit (WDK). However, those too come without a blog post detailing the changes.
Regardless, if you're interested, you can download the latest Windows Server Insider Preview build here, where you can also find the latest preview version of Windows Admin Center. For developers, the Windows 10 SDK Preview can be found here, the ADK Preview is here, and the WDK Preview is here.