Earlier today, Apple announced that it was releasing iOS 11.4 with AirPlay 2 support and Messages in iCloud. What the firm didn't say was that it's also releasing tvOS 11.4 and watchOS 4.3.1. Sadly, macOS 10.13.5 High Sierra remains in beta, likely to be released later on this week.
Most notably, tvOS 11.4 includes AirPlay 2 support, which is designed to allow you to play audio on multiple devices at the same time. You can control AirPlay 2 playback from your Apple TV, as well as from your iOS device running iOS 11.4.
watchOS 4.3.1 doesn't contain much in the way of new features, as the version number indicates. It does fix an issue that causes an Apple Watch to get stuck on the Apple logo when the device boots up.
There are also security fixes involved in all of these updates, although Apple hasn't listed them just yet. When they're available, you'll be able to find them here. For tvOS 11.4, you can update your fourth- or fifth-gen Apple TV via the Settings app, and for watchOS 4.3.1, you'll need to use the Watch app on your iPhone.