Quality Score is a Google Ads' metric that assesses the overall quality of your paid ads. It looks at things like how user-friendly the ads are and how likely they are to drive clicks.
A high Quality Score indicates a successful ad, making Google more likely to give you a better ad placement.
To be clear, Google doesn't use Quality Score as a direct factor in how it ranks your ads. The way your ads appear is determined by Ad Rank, a similar metric.
But Quality Score looks at many of the same factors as Ad Rank, making it a valuable metric to maximize.