Relevance score

Definition:

A rating of 1 to 10 based on how your audience is responding to your ad. This score is calculated after your ad receives more than 500 impressions.  Facebook considers how relevant an ad is when determining which ads to show. When your ad is relevant to your audience, its relevance score is higher and it is more likely to be served than other ads targeting the same audience. As a result, you pay less to reach your audience.