tvOS 12.2.1 is a free download version that can be downloaded over the air through the Settings apps on the Apple TV by going to System -> Update Software. Apple TV the owners who own automatic software update will be automatically updated to tvOS 12.2.1.
It is not clear what was set up in tvOS 12.2.1, but because it is a small 12.x.x update, it is likely that it will address bugs and other issues found in tvOS 12.2.
Apple does not issue clearance notes for tvOS updates, making it difficult to find out what may be new or improved in the operating system.
The prior update, tvOS 12.2, introduced new AirPlay 2 features, allowing users to request Siri to play specific media. Apple TV or third-party TV (support soon) from an iOS tool.