PreBoard is essentially a built in 'safe mode' for tvOS that has existed since at least tvOS 11, if not sooner. I believe it was introduced around the same time the 4K came out to try and help mitigate issues to prevent people from needing to return the 4K's when things go awry.
How it Works
PreBoard tracks 'BootCount' inside this specific file "/var/mobile/Library/Caches/com.apple.PreBoard/BootCount". Every time either backboardd or PineBoard crashes (i'm not sure if its just one of the two or both) This count is incremented, as soon as it >= 4 The next time backboardd/PineBoard are killed or they crash instead of PineBoard showing up to the user they will get presented with PreBoard.