tvOS 12.2.1 is a free update that can be downloaded over the air through the Settings app on the Apple TV by going to System --> Software Update. Apple TV owners who have automatic software updates turned on will be upgraded to tvOS 12.2.1 automatically.
It's not clear what's been fixed in tvOS 12.2.1, but given that it's a minor 12.x.x update, it likely addresses bugs and other issues that were found in tvOS 12.2.
Apple does not provide release notes for tvOS updates, making it difficult to determine what might be new or improved in the operating system.
The prior update, tvOS 12.2, introduced new AirPlay 2 features, allowing users to ask Siri to play specific media on an Apple TV or third-party TV (support coming soon) from an iOS device.