He was talking about the retrospective prime directive:
Regardless of what we discover, we understand and truly believe that everyone did the best job they could, given what they knew at the time, their skills and abilities, the resources available, and the situation at hand. --Norm Kerth, Project Retrospectives: A Handbook for Team Reviews
I always found useful to read it loud at the beginning of a retrospective. In fact, I never regret having read it. It is quite the opposite. I could recall the few times I regret not calling it loud: the vibe was a little off.
Let’s participate on this ceremony –the retrospective—with this common belief. It is more than considering people did their best. It is a belief! I don’t ask people to hold this belief forever (people do make judgments about skills, abilities and situations). I do ask people to hold this belief during the retrospective time.
Here is the premise of the retrospective premise (prime directive):
During this retrospective, only for the next X minutes, let’s adhere to this common group belief, and find out where it will take us... Regardless of what we discover...