The Ad Rank Threshold is the minimum price, or the reserve price, that a bid needs to meet to be displayed. If there are no other bids on the auction, then the winning bid pays the reserve price.
Thresholds are dynamically calculated at the time of each auction based off of: Ad position and the quality of the ad, the user’s context, the topic of the search, and other related auctions.
PPC Management Agency Google Adwords Metrics Ad Rank Threshold