Ad Rank is a Google Ads metric that decides whether an ad shows and in which position. It is calculated mainly from the cost-per-click bid and the quality of the ad and landing page (Quality Score), taking the query context into account. In practice this means a higher position can be achieved not only with a bigger bid, but also with a better, more relevant ad: good quality lowers the cost for the same position. (Compare: Quality Score, CPC)