Update 08/08/2019: We replaced relevance score with ad relevance diagnostics to make diagnosing ad relevance clearer and more actionable.

Facebook introduces relevance scores in ads reporting, calculated from expected positive/negative feedback from target audience.
Scores range 1-10, updated with interactions.
Higher scores reduce delivery cost by signaling ad relevance to the system.
They aid in testing ad creatives before campaigns and monitoring ongoing campaign health.
However, relevance score should not be the primary performance indicator; businesses should prioritize achieving their advertising goals (e.g., sales).
Use scores to lower costs, test and learn, but remember a good score is not an end itself.
Score rollout begins this week in Ads Manager and API.