As is usual for tvOS updates, the release notes on your Apple TV say next to nothing. The security patch notes document that accompanied the release specify that the following issue was fixed:
Available for: iPhone 5s and later, iPad Air and later, and iPod touch 6th generation
Impact: A malicious application may be able to execute arbitrary code with system privileges
Description: A use after free issue was addressed with improved memory management.
CVE-2019-8605: Ned Williamson working with Google Project Zero
You can download the update through Software Update on your Apple TV.