Ad Strength is Google's feedback score (Poor to Excellent) on how relevant and varied your responsive search ad assets are. It is guidance to improve an ad, not a ranking factor in the auction.
What it grades
Ad Strength looks at whether you have enough headlines, enough variety, and enough keyword relevance in a responsive search ad. It nudges you toward supplying the raw material Google needs to test combinations well.
Do not chase the label
Excellent Ad Strength does not guarantee performance, and a Good ad can outperform an Excellent one. Treat it as a checklist for asset variety, then let real metrics, CTR, conversion rate, and CPA, decide what actually works. It is not part of Ad Rank.