These mutually exclusive commands either hide or show function curves for tracks that are not highlighted in the Controller window.
When Hide Non-Selected Curves is active (the default), curves appear in the Key window only for tracks that are highlighted in the Controller window.
When Show Non-Selected Curves is active, curves appear in the Key window for all tracks that are visible in the Controller window.
When Show Non-Selected Curves is active, you can also use Freeze Non-Selected Curves. That way you can see curves for non-highlighted tracks but not edit them inadvertently.