A deep dive is a thorough examination of a single topic: the session where a team goes past the summary slide into the data, the edge cases and the awkward questions. The diving metaphor is exact: you go down slowly, stay a while, and surface with things you could not see from the boat.
Good deep dives are scheduled, scoped and rare; when everything is a deep dive nothing is. In workplace management the classic one is the utilisation deep dive that finally answers whether the office is full or just feels full on Tuesdays.