The macOS High Sierra 10.13.6 Update adds AirPlay 2 multiroom 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.

See Apple Security Updates for detailed information about the security content of this update.