Earlier this week, Microsoft released Windows 10 Insider Preview build 19608 to the Fast ring. There wasn't a whole lot that's new in the build, but it did have some issues. Presumably, that's why the company is releasing build 19608.1006 to the Fast ring today.
According to the blog post, it only has a single fix, and it's the "improve overall reliability of the OS". Unfortunately, the firm didn't specify exactly what got fixed, so if you were struggling with an issue with the latest build, you can install the new cumulative update and take your chances to see if it's fixed.
While the Fast ring is always for the development branch, this news comes on the heels of Windows 10 version 2004 being promoted to the Release Preview ring. That means two things. One is that the next feature update for Windows 10, the May 2020 Update, is going to be released to the general public soon. The other thing is that Windows 10 20H2, presumably build 19042, will be offered up to the Slow ring soon.
You can install today's cumulative update via Windows Update. If you're not on the Fast ring yet, you can enroll via the Windows Insider Program tab.