All content is accurate for 3.67 Frontiers.
Ambient is a hidden mode where the game plays itself; cycling through different planets & star systems; like different scenes in a movie. It's great for if you love the visuals & music of the game; but just want to sit back & relax rather than play. It's also good for falling asleep to.
To access it, you need to change one of the first lines of code in your save file; which shows the game mode you are playing in, using a unique number for that version of the game. Figuring out which number represents ambient mode in the current patch of the game should be fairly easy; just add 512. For example, in July 2020 it went: normal: 4638 + 512 = creative: 5150 + 512 = survival: 5662 + 512 = ambient: 6174 + 512 = permadeath: 6686