A Postmortem or Retrospective is a reflection on past actions with an eye to what went well, what didn't, and the cause of any failures. Retrospectives are crucial for improving one's rationality: they are the opportunity to the grade both one's direct decisions as well as the decision-procedure and epistemic algorithms that one was employing. Sharing such accounts publicly is prosocial and allows others to learn from one's experience too.
One can ask "could I have done better had I reasoned better with the information available?" Often the answer is "yes", and one can apply lessons learnt going forward. It can feel painful to reflect on one's mistakes, but doing so is how one grows.
This tag is specifically reporting actions and outcomes together with an evaluation of the choices/thinking patterns used. If a post focuses on changes in general beliefs about the world, without reflecting on specific actions, then it is a good fit for the Updated Beliefs tag. A central example of a Postmortems & Retrospectives post is “Arbital Postmortem”; in contrast, central examples of Updated Beliefs posts are “'Other people are wrong' vs 'I am right'” and “Six Economics Misconceptions”.