Previewed at WWDC, launching in the fall.
Apple Seeds First Beta of macOS Sierra 10.12.6 to Developers
macOS Sierra 10.12.6 can be downloaded through the Apple Developer Center or using the Software Update mechanism in the Mac App Store.
We don't know yet what's included in the macOS Sierra 10.12.6 update, but with the debut of the next-generation version of macOS approaching, it's likely to feature minor bug fixes, improvements, and security enhancements.
Development of macOS is wrapping up as Apple transitions to macOS 10.13, which will be introduced at the Worldwide Developers Conference in June.