What is Ad Rank?

Ad rank determines your ad position on Google page, and it is determined by your bid for that keyword and quality score.

Ad Rank determines the position of a pay-per-click add in relation to other ads on a search engine results page. Ad rank is given by measuring your ad relevancy to landing page and how that ad  is related to people who are searching for that keyword.

The position of this ad depends on the position of each keyword included in the ad, which is determined by your bid for that keyword or ad group multiplied by the matched keyword's Quality Score.

All other things equal, the more relevant your keywords, ad text and landing pages, the higher your Quality Score, and the higher your ad ranking.
Ad Rank = Quality Score * Maximum CPC with Ad extention