What it is
A diagnostic signal from Google, not a bidding input.
Quality Score is Google’s way of telling you how well your account fundamentals are working. It is not a lever you adjust directly. It is the scoreboard.
The score is assigned at the keyword level and drawn from three components: expected click-through rate (based on the historical performance of the keyword), ad relevance (how closely your ad copy matches the search query), and landing page experience (page speed, content relevance, mobile usability, and trust signals). Each component is rated below average, average, or above average, and the combined signal produces the 1–10 score visible in the Google Ads keyword report.
A keyword with a low Quality Score is a keyword you are paying a premium to run. A keyword with a high Quality Score is a keyword Google is effectively subsidising, because you are producing the kind of ad experience Google wants to serve. That asymmetry — the same bid producing radically different actual costs — is the entire commercial point of the mechanism.