Update: Sources say a newer cumulative update has been compiled; build 10586.312 that will likely head to the public instead. Original article follows, just replace all 10586.306 references with .312.
As Microsoft continues work on the upcoming Windows 10 Anniversary Update, a smaller team within the company is focused on providing bug fixes and updates to the latest current branch build of Windows 10, which is build 10586. Right now, Microsoft is testing another new cumulative update that may roll out to the public very soon; build 10586.306.
Much like previous cumulative updates before it, build 10586.306 includes mostly under the hood improvements, bug fixes and performance tweaks. Build 10586.306 also improves the overall experience of the operating system.
This update could roll out to the public very soon (assuming a newer build isn't compiled before then), as the most recent cumulative update compiled was only sent to Insiders on the Release Preview ring, and was not delivered officially. Build 10586.306 includes improvements made in 10586.240 and 10586.242, plus more. The update, if it does reach the public, will likely roll out for Windows 10 Mobile and Windows 10 desktop users.
The Windows 10 Anniversary Update is just around the corner. With a launch date of July, there’s only about a month or so left before the Anniversary Update is signed off. The next few weeks should see Microsoft finalise and lock down the feature set of the Anniversary Update, for public release in late July.