From Microsoft there's a FAQ:
Q: Who is receiving this build?
A: This release is being sent to Retail open market devices as well as our Insiders in the WIP Slow and Release Preview rings.
Q: I'm in the WIP Fast ring. How do I obtain this .164 release?
A: Change rings in the Windows Insider app to either WIP Slow or Release Preview, take the update, and then change back to the WIP Fast ring.
Q: Why is it necessary for WIP Fast users to switch rings?
A: It's a publishing limitation. Changing rings will not be a permanent process, but it is required for this particular release.
Q: Why didn't Insiders get to test this release before releasing it to Retail users?
A: Insiders did get to test it! The previously released 10586.122 build is nearly identical to this new .164 build. There are no major feature changes between the two builds. Changes were focused on a few key bug fixes and other required updates to ensure this build was ready to release to Win10 Retail devices.
So a minor release for anyone already on the Slow or Release Preview rings, but an important release because it's also available to all native Windows 10 devices and is thus a line in the sand going forwards. And, I strongly suspect, the version of Windows 10 Mobile which will get rolled out to the likes of the Lumia 640 and 735 (etc.) packaged as an official over the air update.
As usual, head for Settings/Update & Security/Phone update and then 'check for updates'. We're installing this on many of our test devices at the moment.
The 'for all' in the headline refers also to anyone with a WP8-native device which has been placed on the Insiders programme, of course. If this is new to you then just install the 'Windows Insider' application from the Store and then accept the terms to go on the 'Release Preview' ring. Good luck!