What Is Quality Score and Why Does It Matter?
Quality Score is an estimate of how relevant your ads, keywords, and landing page are to a person seeing your ad. Having a high Quality Score means that our systems think your ad, keyword, and landing page are all relevant and useful to someone looking at your ad. Having a low Quality Score, on the other hand, means that your ads, keywords, and landing page probably aren't as relevant and useful to someone looking at your ad.
For more info read here; http://www.seoinc.com/seo-blog/what-is-pay-per-click-quality-score/
Quality score is a rank of how relevant and useful a search engine thinks your business is to someone searching for your products and services online, and this rank impacts both the cost and position of your pay-per-click ads on the search engine results page.
Search engines evaluate a variety of factors when determining quality score, such as your keyword clickthrough rate (CTR) history, the quality of your landing page, the relevance of your text ad, and the relevance of the keywords you are bidding on. Google has recently addressed some of these factors that help determine quality score.