At any given time, you compete against similar businesses for a share of the Facebook ad audience. Naturally, the platform won't be able to show everyone's ads all the time. So, it relies on an algorithm to determine which ads are more likely to be relevant and interesting to users and thus deserve more screen time.
Facebook's Ad Quality Ranking evaluates your ad against other ads that are competing for the same audience and assigns it a quality ranking. This ranking is based on various factors, including the following:
Facebook users who have hidden your ad after seeing it
The number of low-quality attributes found in your ad (more on that below)
The relevance of your ad versus other ads being shown to the same audience
User engagement with your ad, such as clicks, comments, and shares