What does it actually mean to be psychologically safe at work?

Psychological safety is cited everywhere and genuinely practised rarely. Here is a more concrete account of what it actually is, what it feels like, and what it requires.
What psychological safety actually asks of a team

Psychological safety isn’t comfort or niceness. It’s the conditions that allow people to contribute, challenge and repair; and it’s built at the level of the nervous system, not through culture programmes.