AirPlay users will want to install this update sooner than later.
MacOS High Sierra 10.13.6 includes AirPlay 2 multi-room support, so you can now play music on multiple AirPlay 2 speakers and control them on the Mac.
According to the release notes, the 10.13.6 update also:
Fixes an issue that may prevent Photos from recognizing AVCHD media form some cameras
Fixes an issue that may prevent Mail users from moving a message from Gmail to another account
The update also includes a number of security updates that Apple details in a
How to install the macOS High Sierra 10.13.6 update
Before updating, perform a backup of you Mac. Then follow the steps below.
1. Click on the menu, select About this Mac, and then in the Overview section, click the Software Updatebutton. Or you can launch the App Store app in your Applications folder.
2. In the App Store app, click on Updates at the top of the app.
3. An entry for “macOS High Sierra 10.13.6 Update” should appear. If you don’t see it, click on Store > Reload Page or press Command-R.
4. Click on the Update button to the right of the entry. The update will take several minutes and your Mac will need to restart.