Quality Score (QS) is a system used within AdWords to determine how relevant your ad content and keywords are to the search query. Your quality score is given to you on a keyword level basis, also affecting your overall ad group campaign performance.
Your quality score is something you should care greatly about. With having a great quality score, comes a cheaper price to rank for a comparative position. The lower the quality score, the more you will have to pay. Also, the higher the quality score, the lower the cost per click (CPC) bid. Your QS may be the difference between having to bid $3 or $1 per click.