Ad Rank determines whether your ads are eligible to show & if eligible, where on the page your ads are shown (if at all) relative to other advertisers’s ads.
Calculated based on many factors, including bid amount, the quality of ads & landing page, the Ad Rank thresholds, the competitiveness of an auction, the context of the person’s search & more.
In Google Ads, it can be calcuated using: CPC Bid * Quality Score.