The macOS High Sierra 10.13.6 update adds AirPlay 2 multi-room audio support for iTunes and improves the stability and security of your Mac. This update is recommended for all users.
AirPlay 2 for iTunes
● Control your home audio system and AirPlay 2-enabled speakers throughout your house
● Play music at the same time on multiple AirPlay 2-enabled speakers in your house, all in sync
Other improvements and fixes
● Fixes an issue that may prevent Photos from recognizing AVCHD media from some cameras
● Fixes an issue that may prevent Mail users from moving a message from Gmail to another account
For more detailed information about this update, please visit: https://support.apple.com/kb/HT208864
For detailed information about the security content of this update, please visit: https://support.apple.com/kb/HT201222
You can download the macOS High Sierra 10.13.6 update from the Updates tab in the Mac App Store.