It's basically a meeting between writers, directors and producers, to come out with the story structure and main points. The UK Writers Union's "Showrunning in the UK" guide defines it as a "brainstorming session where non-writing producers will ultimately make the creative decisions [...]. In the UK, it’s commonly used in producer-led continuing drama."

I honestly don't know how common it is to record it, rather than just having minutes of it written down by some assistant.