what is quality score?

Quality score determine by google adwords .Quality Score scale 1 to 10.

the components of Quality Score (expected clickthrough rate, ad relevance, and landing page experience) are determined every time your keyword matches a customer's search.

• You can get a general sense of your ad quality in the "Keyword Analysis" field of your account (reported on a 1-10 scale). You can find this by selecting the Keywords tab and clicking on the white speech bubble next to any keyword's status.
• The more relevant your ads and landing pages are to the user, the more likely it is that you'll see higher 1-10 Quality Scores and benefit from having higher quality components of your Ad Rank, like a higher position or lower CPC.