Quality Score is an estimate of the quality of your ads, keywords and landing pages. The higher your quality score, the better your Ad Rank (better ad position and lower cost-per-click).
Quality Score is decided from the expected CTR (click-through rate), ad relevance and landing page experience. If your ad is not relevant to the keyword, you will get low CTR and ultimately a bad Quality Score so make sure your ad copy is relevant to your keywords and create good landing pages.
Here are some ways to improve your Quality Score: