The Facebook Relevance Diagnostic (previously known as Relevance Score), tells you how relevant your target audience finds your ad to be.
When ad relevance is high, it is a win for both the audience and the business – the audience is seeing something that adds value to them and the business gets better ad performance at a lower cost.
It is recommended that you use the Relevance Diagnostic in conjunction with other metrics as it may give more context about an ad. Eg: If your ad is not performing well, look at the Relevance metric to see if numbers are low. If that is the case, it may indicate that your ad is performing poorly because your audience does not find it relevant and that you need to change up the message or creative.
That said, if your ad is achieving your objective (purchase, conversions, etc) but the relevance scores are low, you can disregard it as your ad is still working well and achieving your ad objective.
The different Ad Relevance metrics are:
1. Quality Ranking
2. Engagement Rate Ranking
3. Conversion Ranking
These can be found in your Facebook Ads Manager Dashboard.