Quality score will ultimately determine which ads are eligible to enter an auction and affect whether they are displayed for a certain term. It mainly affects three areas for an ad: if the ad can enter the auction, how the ad is ranked in the auction and what the actual CPC the advertiser will pay.
If an advertiser’s ad is written and targeted on a keyword which has a high quality score the ad is then able to be entered into more auctions. Whereas a keyword which has a low quality score results in very few chances to be entered into auctions, if at all.
Once the ad is in the auction it is then ranked compared to the competition. This ad ranking is determined by the max CPC (how much the advertiser is willing to pay per click), the quality score of the ad and factors that may increase the relevancy e.g. extensions.
Therefore campaigns that have ads with low quality scores are more likely to find that their ads are not being shown in auctions resulting in very few impressions and difficulty resulting in conversions.