What does this icon mean?
What does above indicate on my AppleTV?
How do I fix it?
Question marked as
Picture-in-Picture.
Currently it works only within the Apple TV app.
When activated, you should be able to see two streams at the same time, one little and overlaid in the corner of the screen.
Use picture-in-picture on your Apple TV with the Apple TV app - Apple Support
How do I fix it?
Currently, the icon can’t be disabled or hidden, when the extra controls are visible on screen.