An ad with higher rank is usually shown before with lower rank. Our scale for ad-rank ranges from 1-5, 1 being the higher and more desirable value. One of the primary function that determines the ad-rank is the relative bid price or cost per click (CPC) or cost per 1000 impressions (CPM) that an advertiser is willing to pay for an ad. Because of the dynamic nature of the real time bidding auction, the ad-rank is rarely stable. It fluctuates on a daily basis. However, a trend in ad-rank or an average ad-rank is a good parameter to identify whether an ad is doing well.
What do you mean by the rank of an ad?