Apple has seeded the first beta of MacOS Mojave. MacOS 10.14.3 is the next maintenance build of the currently version of MacOS. The beta, along with the first beta of iOS 12.1.2 that was released today, is currently only for developers. The Apple Beta Program public builds have not been released at the time of this post.
As for the contents of MacOS 10.14.3, that isn’t 100% clear. It is mostly going to be a bug fix and maintenance update for sure but Apple often sneaks in a new feature or tweaks a feature on each release. Ultimately what will end up in the update will not be known until it is actually released to the public.
As for what that release will happen, it will likely be mid-to-late January 2019. Apple usually seeds anywhere from four to six beta builds before actually releasing it to the public. That, of course, is subject to change based on any big issues found within the beta cycles.
It is always the recommendation of Average Apple Guy to not install the beta builds on mission critical Macs in your life.