"About the macOS High Sierra 10.13.6 Update
This update is recommended for all macOS High Sierra users.

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.
(See below for iTunes 12.8 update.)

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 below for article on security fixes in this update.)

Download pages for macOS 10.13.6 & Security Update 2018-004 updates:

• Download macOS High Sierra 10.13.6 Update (1.88GB)
  
  
• Download macOS High Sierra 10.13.6 Combo Update (2.49GB)
  
  
• Download Security Update 2018-004 (Sierra)
  
  
• Download Security Update 2018-004 (El Capitan)

Related Apple Articles:

• About the macOS High Sierra 10.13.6 Update
  
  
• About the security content of macOS High Sierra 10.13.6, Security Update 2018-004 Sierra, Security Update 2018-004 El Capitan
  
  
• About the security content of Safari 11.1.2
  (available OS X El Capitan 10.11.6, macOS Sierra 10.12.6, & macOS High Sierra 10.13.6)

If you're running Security/AV software on your Mac, it may interfere with updates. Of course backup your current boot drive before updating. (And a bootable clone is a plus.)

FYI on Nvidia Web Driver Updates:
Users of Nvidia's web drivers should know that new OS updates (versions or later build numbers after Security Updates) break any currently installed Nvidia web graphics driver. I'd switch to the default Apple driver and reboot before updating, if your card is supported with the default drivers. If not, wait for a Nvidia driver update. (Some non-Mac EFI cards are not supported by default OS X drivers.)
Of course have a current backup (and if possible, a Clone) of your boot drive before any update. (As usual, I'll post download pages for Nvidia updates when spotted in the news and video topics page.)

Nvidia Web Graphics Drivers Updates:
On July 10th and 11th Nvidia posted driver updates for macOS 10.13.6 and new builds of 10.12.6/10.11.6 after installing Security Update 2018-004.
 

• Download iTunes 12.8
  (FYI - some iOS users reported problems with iTunes 12.8 & macOS 10.13.6.)
  - iTunes now supports AirPlay 2 multi-room audio and HomePod stereo pairs.
  - Use the AirPlay menu to easily group HomePod, Apple TV, and other AirPlay 2-enabled speakers and control what plays next.
  
  
• iOS 11.4.1 release notes
  - Improves reliability of syncing mail, contacts and notes with Exchange accounts.
  - Fixes issue that prevented some from viewing last location of their AirPods in Find My iPhone.
• About the security content of iOS 11.4.1
  (Also see Doc on using USB Accessories w/iOS 11.4.1 below.)
  
  
• tvOS 11.4.1 release notes
  - This update includes general performance and stability improvements.
• bout the security content of tvOS 11.4.1
  
  
• watchOS 4.3.2 release notes
  - This update includes improvements and bug fixes.
• About the security content of watchOS 4.3.2