Relevance diagnostics are Meta's three per-ad rankings, quality, engagement rate, and conversion rate, that show how an ad compares to others competing for the same audience. They point at whether a weak result is a creative, engagement, or offer problem.
The three rankings
Each is shown as a range (below average to above average) against ads chasing the same people: quality ranking (perceived quality), engagement rate ranking, and conversion rate ranking. They are diagnostic labels, not scores that directly set cost.
How to read them
The value is in which one is low. Low quality or engagement points at the creative; low conversion ranking points at the landing page or offer, not the ad. They are the Meta cousin of Google's Quality Score components, useful for locating the problem rather than as a number to chase.