The new macOS Mojave 10.14.6 beta can be downloaded through the Software Update mechanism in System Preferences after the proper profile has been installed from Apple's Developer Center.
We don't yet know what improvements the next update to macOS Mojave will bring, but it is likely to include bug fixes and performance improvements for issues that weren't able to be addressed in the macOS Mojave 10.14.5 update.
macOS 10.14.6 is likely to be one of the last updates to the macOS Mojave operating system as Apple will soon transition to work on macOS 10.15, the as-of-yet unnamed next-generation version of the macOS operating system.
Apple will debut macOS 10.15 at the Worldwide Developers Conference, set to kick off on June 3. While it will be in beta testing for several months before seeing a fall release, work on current software often slows down as Apple gets ready to launch new software.