Apple has released macOS High Sierra 10.13.6. It adds AirPlay 2 multi-room audio support for iTunes and improves the stability and security of your Mac.
It also fixes an issue that may prevent Photos from recognizing AVCHD media from some cameras and fixes an issue that may prevent Mail users from moving a message from Gmail to another account.
macOS High Sierra 10.13.6 can be downloaded at the Mac App Store or via the Software Update function in the Mac App Store.