Quality Score is a score given to each keyword in an adwords campaign. The quality score is a “measure of how relevant your keyword is to your ad text and to a user’s search query.”
Here’s what Google says:
“The AdWords system calculates a ‘Quality Score’ for each of your keywords. It looks at a variety of factors to measure how relevant your keyword is to your ad text and to a user’s search query. A keyword’s Quality Score updates frequently and is closely related to its performance. In general, a high Quality Score means that your keyword will trigger ads in a higher position and at a lower cost-per-click (CPC).”