Each keyword is assigned a dynamic variable called Quality Score. This Quality Score is a measure of how relevant your keyword is to your ad text and to the user’s search query. To calculate this variable a variety of factors needs to be considered. The Quality Score formula usually depends on whether it’s calculating minimum bids or assigning ad position.
The Quality Score influences your ads’ position on Google or the Google Network. It also partly determines your keywords’ minimum bid. Usually the higher you’re Quality Score the better your ad position and the lower your minimum bid.