Just hours after Apple released iOS 11.4.1 today, Apple also released an update for macOS. This update adds iTunes support for AirPlay 2, as well as many other mentioned and unmentioned bug and security fixes.
This may very well be the very last version of High Sierra to be released before Mojave comes out this fall. Once Mojave is released it will be difficult to get a copy of High Sierra if Apple follows their historical pattern. If you are still running an older version of macOS I strongly recommend you update now if you can. At the very least you should download the installer from the App Store but close the installer when installation starts so that you have a copy you can upgrade to in the future. El Captian will stop receiving security updates this fall, Sierra will end security updates in the fall of 2019, and High Sierra in the fall of 2020.
To update your Mac go to the Apple menu > App Store > Updates.