Quality Score and maximum CPC bid, i.e. the relevance of your ads and the respective marketing budget, contribute to the calculation of AdRank. AdRank ranks your ads against your competitors' ads.
Your AdRank depends on your competitors and is constantly recalculated, whenever your ad is displayed. This might frequently cause slight fluctuations of your AdRank. The higher your AdRank, the more customers actually notice your ad, the better for your campaign.